Just bought myself a PSP

I just bought myself a PSP Slim with custom firmware on eBay, but the auction site removed most of the listings due to a strange reason. Luckily I bought it and it shipped out before the listing was closed. UPS will probably arrived at my doorstep Monday or Tuesday. The problem is that it does come with a memory stick, so I'll either have to buy it online or buy it at a brick & mortar store. I need suggestions on what kind of memory stick should I be buying and at what place.