Unless your car has been converted to 134a, you've got R12. You can't buy R12, but you can have it recharged somewhere. R12 is very expensive though. I'd take it to a place that'll do a fairly inexpensive or free system check to figure out what the problem is. If you've got a leak and they need to replace the seals, you'd might as well have them convert it to 134a, as it will likely be cheaper than replacing seals and putting R12 back in. They should give you credit for any R12 they remove. If you just want to squirt something in there to get you by and on't care if it leaks back out, there are R12 equivalents that are a lot cheaper than R12 (but more expensive than R134a). You could put in a can of the cheaper stuff. Mine doesn't work either, it cools but not anywhere near what it's supposed to. So I'm going to have it checked out sometime. If you're interested in the R12 equivalents check out www.autofrost.com and www.refrigerantsales.com. Whatever you do, make sure that any place you take it to guarantee's their work, you don't want to have to come back next summer.
There are a lot of things to check before you just add freon. Check fuses. Check a/c relay. Pressure switch bad? Is your compressor kicking on? There isn't only freon problems. Sometimes if the compressor isn't kicking on your compressor clutch may need to be checked with a feeler gauge to see if it is out of spec. Then usually all you have to do is take off the clutch and remove 1 of the shims. You may be dissappointed if you spend the money on freon and it doesn't need freon. Hope this helps.
Mine still blows cooler air, so I know it's low, but not low enough for the pressure switch to cut it off. I'm just taking mine in, and for $30 they'll figure out what it needs to be fixed and how much it'll cost. If I decide to have them do the work, they credit me the $30 towards that. So I'm not messing with it, besides the fact that I don't have R12 or 134a gauges nor the equipment to pull a vacuum on it to check for leaks.
BEFORE you add freon you should
1 Check that the a/c clutch is operating properly
2 Check that the a/c condensor fan is operating properly (BOTH fans should run when the a/c is on)
3 Check that the low side line and make sure that it is cool ( Touch the line to tell)
If all these are operating properly then you would probably want to go to your local Nissan dealer or a reputable shop to check your freon pressures to see if you need freon. If you have any questions feel free to e-mail me at Ray Brandt Answer Guys