I get mine from 18004memory.com. (guess what their phone number is...)
I don't know where our store gets their memory, but they're a bit of a con. I tried to get memory for my mac pro (4gb) and my "at 5% markup from wholesale" was significantly more expensive than 1800's was. I actually called them up and asked what the deal was, and he says but our memory's guaranteed. Ya, isn't it all nowadays? I wouldn't buy memory that didn't come with a lifetime warranty from anyone. They know though, if it fails a year from now, it'll cost them a tenth of what it originally was to replace, the shipping will be the big expense if they have to replace it. And by then you'll probably just want to replace it with bigger memory anyway. So quality and warranty have very little to do with memory nowadays.
I work for the Department of Redundancy Department