The internal circuit breaker keeps tripping. Have to reset every time. Where can i get a new one?

The unit's circuit breaker is there in case of internal faults in the uinit. The best option is to contact the dealer from whom you purchased it or the manufacturer.

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

Replace the limit, by tripping often they become even more likely to nuisance trip.

All resets and breakers trip with high heat.
Do you mean the tankless unit is tripping the 40 amp circuit breaker?
Or the thermal cut-out, located on unit, is tripping?

Is this a new installation?
Or was the tankless working before and stop working?

Copy following link to check wiring.
AE 9.5 requires 40 amp breaker and 6 gauge wire.
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

If the unit is tripping the thermal cut, -out, then product manual says to call Bosch or service technician.
Call Bosch 866-330-2729
http://waterheatertimer.org/pdf/Bosch-Powerstar-AE3.4-7.2-9.5.pdf
Thermal cut-out can trip if water flow is too low.
Try changing power selector screw from Hi to Lo so only 1 element is ON. See diagram page 6 of manual.
Adjust the 'outlet service valve:' The manual recommends that a ball valve be installed between hot outlet and faucet. Adjust the valve so more water flows to faucet.
Remove aerator, clean aerator, don't use low flow restrictions or low flow shower heads, replace leaky faucet cartridge on single-handle faucet.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

It can be the breakers in your house or the reset button is malfunctioning

If the circuit breaker will not reset it could be a few different things. First, circuit breakers do wear out. The bi-metal strip that makes the breaker function does break down metallurgy wise. A new one would have to be installed to fix this problem. The other possibility would be that a wire inside the water heater has grounded out thus not allowing the breaker to reset. A wire might have gotten to hot and the insulation around it might have melted or became brittle and fell off allowing the wire to touch bare metal inside the water heater.

Breaker could be bad.
Let's do a quick check and save you money.
Open top cover on water heater and push in red reset button that might be tripped.
If reset is tripped, water heater should turn ON.
If reset will not reset, then replace upper thermostat:
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
If reset is not tripped out, then it is OK, so test two top wires on upper thermostat.
Use cheap multimeter from home center, and set dial to 240-250-277 VAC.
Test across top two screws on upper thermostat to see if you have 240Volts.
To do full troubleshoot on ordinary tank electric water heater takes 30 minutes and multimeter
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

If water heater is not receiving 240V, then the breaker is suspect, or a wire between breaker and water heater is loose.
See basic water heater circuit
http://waterheatertimer.org/240-v-water-heater-circuit.html

Now let's look at circuit breaker:
Mark suspect 240V breaker.
Identify all your breakers, and also mark another 240 Volt breaker that is same amperage as water heater 240V breaker.
Turn off main breaker.
Take front panel off breaker box.
The wires and lugs on main breaker are HOT, so stay away from that area.
http://waterheatertimer.org/See-inside-main-breaker-box.html
Suspect breaker has 2 wires coming from breaker.
See if one or both wires are loose, and then tighten.

If wires are tight, then move these wires to the other 240V breaker that is rated for same amps as water heater.
Now turn on main breaker.
Put ear against water heater tank to hear bubbly fizzing sound.
This will identify if circuit breaker is bad, or if water heater needs a full troubleshoot.
It is not a good idea to run two 240V appliances on same 240V circuit breaker, so you may have to decide one or the other appliance until you can replace breaker.
Instructions to replace circuit breaker:
http://waterheatertimer.org/How-to-replace-circuit-breaker.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater circuit.

Electric water heater can be repaired by homeowner.

Four things can cause NO hot water:
1) ECO red reset button tripped. Remove top cover on water heater, push red reset button on thermostat. Put insulation and cover back over thermostat.
2) Circuit breaker is tripped, circuit breaker bad, or wire loose in breaker box. Turn breaker on-off and listen for crackling or fizzing sound, and burned smell. Test breaker for 220Volts. Test top two screws on upper thermostat to see if water heater has electricity.
3) Bad upper thermostat. Replace upper thermostat.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
4) Bad upper element. Test upper element.
http://waterheatertimer.org/How-to-test-water-heater-element.html

It's time to replace your thermostat. Instead of maintaing the correct water temperature, the water is getting so hot it is acivating the safety circuit breaker. If this breaker were to fail, you could have a devastating explosion

I am assuming that everything was fine and then all of the sudden available hot water usage time dropped.

On a dual heating element hot water heater it can be

1. Internal tank loaded with sediment (drain and purge from lower garden hose fitting)

2. Thermostat or element problem with ONE of the 2 heaters. (Trouble shoot element thermostat)

3. Check Circuit breaker for partial trip of the breaker.

Please get an expert in - this is either the breaker is faulty or you have an electric fault int he boiler. You can try and replace the breaker - but match the current rating. But this could be dangerous if you keep trying to reset.