SO I'm not even sure you can buy cans of the stuff. Most of those little jobs are 12 oz or the bigger ones 18 oz I think. 16 oz to the lb, so trying to get it right is a crap shoot, depending on what your system takes.
You can buy it, most stores will have to order it, not an in stock item. Looks like pricing right now is about $90/lb in 8oz cans at most local auto parts. You will also need the manifold with the correct connectors as they are different from the 134a connectors.