You can either get an accelerator+memory OR just a memory expansion card. They install in the trap door located under your Amiga. An accelerator will cost you more $ but probably a better investment long-term, if your just going to play retro-games, WHD, etc. you can get by fine with just a ram expansion.
Your pretty much stuck with the used market (ebay, forums, etc.), as there is nothing currently in production... Google "Amiga hardware database" and you can find some good write ups on the various models. Some are more PCMCIA friendly then others.
I may be selling my DKB1202 w/ 4MB of ram if I can get my accelerator back to life. Private message me if interested.
http://amiga.resource.cx/photos/dkb1202