Make sure you get a set of gauges to go along with it if you do it yourself and read up on how much it takes and how to do it...too much can wreck a system. If its gone and out of R134, you most likely have a leak in the system.
Most cans come with leak seal in them. It usually says on the can. Some auto parts stores will let you borrow a gauge to use, that way you can by the cheaper cans. Really depends on the people working there, not necessarily the chain.