I bought an 8-blocked memory card for 19.99 somewhere. It's basically 8 memory cards in one, dont know how it works.
Most cards of this type use a small processor on the card to compress the data into a contiguous "page". To access the other "pages", the user must press a button on the card to initiate the switch and then wait for the card to decompress the page. On some cards this takes up to 1 minute or so. It's a nice concept considering that you don't have to haul around multiple memory cards if you store a lot of saves, but I've found these cards to "crash" quite often...as in once every few months.

After that happened to me a few times, I switched back to an official Sony card.
Although I don't have one for sale, I would suggest looking for a Dex Drive if you're serious about your saved games. It allows you to back up your memory cards to your PC as a file. I would guess you can find them on eBay for under $20, but I could be mistaken.