Which PS1 Memory Cards work on a PS2 Slim

So I've got a few PS1 memory cards and games that I want to play on my PS2 slim, but despite several years of searching, I have yet to find a memory card that my PS2 will manage (mostly I just need to clear out the crap the previous owners left on it).

Is there some trick, or rule that I'm not aware of that I need to be able to save these?

Solution 1:

If I remember correctly; The Slim version supports only the PS/X(1995-1999)version memory cards, not the PSOne(2000-2500) version ones. This could be your problem.

(I used to have 2 memory cards, the "hard edged" PS/X one and "round edged" PSOne card, and only the older blocky card worked on my slim.)