Where is the low side service port on a 88 caravan? Putting R-134A in

Freon (R-12) is not outlawed, but you do need a certifiation to buy it. The certification is cheap and easy, can even be done online. Freon itself has gotten quite expensive though.

The setup of a 1988 Caravan will not cool as well with R-134 (~80% at best), but if you live in a cooler climate, it might do the job.

If you use a simple 'changeover' or 'conversion' kit, expect to buy a new compressor within a year.

Having said all that, open the hood, and look straight back from the hood release to the middle of the firewall (behind the engine). You'll see a silver (possibly black) block with a large and a small hose. Follow the larger hose to the left till you see what looks like a valve cap on a tire.
That is the low side R-12 fitting, just past it on the same hose, you'll see a screw-in piece that has two wires coming out of the other end. If you dont see that, you're probably in the wrong place.

Unscrew the cap slowly, it's not uncommon for the schrader valve underneath to leak a bit, you can use your thumb to keep from losing too much freon.
If the part underneath the cap has threads on the outside (just like a tire valve stem), it hs not been (officially) converted to R-134.
You will NOT be able to use R-134 if you see threads on the outside. You would need the 'converter' from the R-12 fitting to the R-134 fitting.

If you really have to 'convert' to R-134 as cheaply as possible, follow these seps and you might get 3 to 5 years out of it:
Get an R-134 can tap with a guage on it, and three cans of R-134.
Get a conversion valve set, usually about 11 bucks, and install according to directions. Don't bother with the 'all-in-one' kits.
Drain the existing oil as best you can. (another valve on the compressor itself makes this simpler, using part of a can of R-134 on the low side will force the oil out when you press the valve on the compressor, and don't forget to replace the cap), then use 4 to 6 oz of 'R-134 Oil Charge' aka PAG Charge. (usually this means 3 cans of 2oz oil charge) Then add 1 can of R-134. Start the engine, turn the AC to max, fan on high. The compressor may not start, or may start and kick out really quickly. That's ok for now.
Slowly add another can in, hold the trigger for about 30 seconds, then let go, keep doing this till the pressure gauge shows ~30. When you get close to 30, use shorter trigger pulls.
At around 22 or so, the compressor should start staying on. Leave the can connected, but set it down on a small towel or rag over to the side. Let the Engine/AC run for at least 5 minutes. Check the guage again. Ideally, you want the gauge to read around 35 to 40 at idle.
DO NOT GO OVER 45! You will kill your compressor when running almost immediately at around 60, and anything over 40 will lessen your cooling and compressor life. You will see the pressure go up a LOT when the compressor kicks off, but you only really need to worry about keeping it below 40 when the compressor is running.

This should all take around 30 minutes or so.

This is NOT a long term fix, but if you get most of the old oil out, and correct new oil in, and keep the pressure to 40 or less w/ compressor running, you might get 3, 4 or even 5 years out of the A/C. That's assuming, of course, the AC system isn't already bad, and there are no major leaks. In most states, it is legal to 'top off' the R-134 the same way in case you have a slow leak.

Personally, I've 'heard' there are other refrigerants tht let you skip all this, and are way cheaper, but EPA mandates a swap to R-134 before you can legally use them, and 12 states have outlawed their use anyway. I've also 'heard' that they tend to work better and cool down faster than R-134 or even R-12 systems, but since I might live in a state that has outlawed them I'd be nervous to try them, now wouldn't I?

TAKE VECHICLE TO A GARAGE THAT DO AIR CONDITIONER WORK BECAUSE 1988 MODEL MORE LIKE HAS R-12 FREON. THAT WONT MIX WITH R - 134 REFRIGERANT THAT WOULD DAMAGE YOUR AC SYSTEM.