From what I've read it's pretty much a given that both cards have to be the same Nvidia GPU though manufacturers and clock speeds can be different. And another note, if you use two cards with different ram sizes, you have to use CoolBit to allow the cards to run at the lower of the two ram sizes (ie. 256 and 512, both would only run at 256 in SLI).
There should still be a few around. This is kinda the risk of SLI though. I think nVidia and ATI are much faster about phasing out the old cards these days, just so you don't have the ability to upgrade for so cheap. They want you to buy two high-end cards at the same time, I suppose. It's one of the many reasons why I'm not a fan of the technology.