look on top of the A/C canister thingy (see pic below) There is a sticker on top which say whether you have a R-12(older,outdated A/C system) or R-134a(new, current A/C system). If you have the older R-12 system, also look for a sticker that says that the car was retrofitted with the newer R134a system (as seen to the left in the pic) maybe by a previous owner. If I'm not mistaken the R-134a system may have been installed in those model years.
It is suggested that you have your AC system serviced by a mechanic, but if you just want to top off your coolant that is something a non-trained person can do by just reading the directions on the refrigerant can/kit.
As far as where you can buy refrigerant, R134a can be bought at any store that has an automotive section (Walmart, Kmart, etc...) or specialize in automotive (Pepboys, Murray'*, Autozone etc). However R-12 or Freon can only be sold to licensed mechanic shops (it'* bad for the environment so it was banned in the use of newer model cars. It is not illegal for shop to use freon.
This link has some more helpful AC tips