Where can I buy a replacement photocell for my Malibu 300W low voltage landscape lighting transformer?

SOURCE: my malibu landscape 300w transformer does not want

Check that power pack is getting electricity.
Power pack should make a small humming sound when on.
Scratch a wire across both terminal screws and see if there is spark.
If a bulb burns out, and is not replaced, additional voltage can burn out other bulbs.
Power pack should be unplugged if bulbs are not burning since that can burn out power pack.
Change out a couple bulbs and then plug-in power pack.
Check connections where lights tie into wire.

My power pack was doing same thing, I bought another power pack from Home Depot to check my lights. All but 2 bulbs were burned out. I replaced bulbs and returned new power pack.. and then power pack quit for good a month later.

I found tips to preserve power packs:
1) Match wattage of bulbs with wattage of power pack
-15 each X 20 watt bulbs = 300 watts, so install 300Watt power pack
2) Installing a 500W power pack on 300W of bulbs will burn out bulbs and power pack.
3) Replace bulbs as soon as they burn out or other bulbs will burn out and power pack will burn out.
4) When working on bulbs, unplug power pack or it may burn out.
http://www.malibulights.com/guides_support/low_voltage_planning_guide/select_your_power_pack

SOURCE: I bought a Malibu 300W

I don't know if it burned out, but something is not right.
Malibu products come with 1-5 year warranty.
New product that stopped working should be exchanged for full replacement at place of purchase, except possibly the auction sites.
New Malibu product should work for many years.
Be sure to match bulb voltage with transformer voltage to avoid possible problems.
Mismatched bulb and transformer wattage should not affect a timer in short period such as 1 week.
Another thing to check is outlet polarity. Use circuit analyzer from home center to check outlet.
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html