Midwest has them too
I actually just ordered both rear arms from Midwest over the weekend while they were running their promotion. Midwest's are cheaper than Vick's, but they're used, but they offer a lifetime warranty on them. So it depends on what you're looking for, but that was good enough for me (nothing against Vick's!).
I looked for ball joints for a long time and eventually decided to go with the whole arm from Matt because of how much trouble I heard it was to replace them and how hard it is to find the ball joint, or expensive or suspect they are when you do. With that said, I remember Bernice posting a link to some aftermarket ones, I think they were for a Ford, that she said would work for the X. I don't think I have it saved though, maybe try a search...
Unless you're really into that kind of work I'd just save yourself the trouble and get the whole arm. That's just how I look at it anyway. Good luck!
