I think every console owner has experienced a time where they came started playing a new videogame, made it to a save point after a daring event, and, to their dismay, experienced a full memory card.  But where can you buy those pesky things the cheapest?  Well, it depends what you're looking for.

If you want, no doubt about it, a brand new, shiny ps2 memory card, I'd have to say the local retail store, or Walmart in general, would probably be your best bet.  I'd like to give a shout out to Bestbuy as well, because they probably offer a more diverse selection of colors and generation of memory cards, just in case you want an original ps1 memory card.

Ok, ok you just want a cheapie?  Well, look no further then Ebay or Amazon, with links even included on this website.  Depending on the quality you're looking for, you can find it online for sure.  For the cheapest storage device possible, just roam the pages of Ebay and set different search options for the best result.

I'd have to recommend, however, getting the best bang for your buck by getting just a slightly used memory card.  Amazon and Ebay both will probably offer some good memory cards that will "git ir dun", so to speak.  After all, as long as it works fine, does looks really matter?  Most slightly used memory cards will probably not look and perform any different then brand new ones anyways.

I'd like to note that many retailers offer brand new products online on sites like Ebay and Amazon as well.  Basically all you have to do is decide if you want a new or used memory card, how much you're willing to spend on shipping and handling, and purchase the dang thing!  I currently have 3 ps2 memory cards and about 6 ps1 memory cards (ps1 memory cards purchased online) that have not failed me yet, 5 years of gaming and on.  I hope you find what you're looking for and happy gaming!