You can buy plasma cutters online and in the real world. If you are just starting out in the buying process the best thing to do is find out exactly which brand, make and model number best suits your needs. Then you have some ammunition when a sales person starts asking you questions. The make, model, and brand of plasma cutter that you choose will depend on the type of cutting you will be doing, as well, as the type of metal and the metal thickness.
Even if I am going to buy the plasma cutter in the real world I always make sure that I cruise the online forums first. There are many people who discuss they types of plasma cutters they bought, why the like them (or do not like them) and where to get them. Even Amazon, which allows customer reviews, will be a value source of information. Then there is YouTube. There are tons of videos about plasma cutting and a couple plasma cutter reviews as well. There are even videos of plasma cutter competitions to see which one is better. You can visit sites like NextTag to comparison shop the various plasma cutter models. Not only that, but there are always plasma cutter auctions on eBay, so check them out too. There are lots of sources of information for your research. It is best to educate yourself before you end up buying something that does not fit your needs.