The little 4 piston ones seem to turn up on craigslist somewhat often at least, maybe keep an eye out on there?

My 8 piston ones were a little harder to come by, but it all worked out.

(I lucked out on getting some new 4 piston Brembos that were off an SRT8 from here a little while back actually and those are what the 8 piston ones replaced)