While you might find a link for the preliminary build for the Epic 4G, it sounds like you might want to wait. The lucky recipients of the leak say there is still work to be done. The apparent fault is a hit-or-miss radio, so essentially the last thing you want to fail on your device. By comparison, a faulty clock on the iPhone sounds pretty manageable.
The Samsung Vibrant, on the other hand, has a reportedly stable Froyo build. While you won't likely find the build, there are two custom ROMs available from the XDA Developers' Blog (see below).
Samsung Vibrant Android 2.2 ROM 1
Samsung Vibrant Android 2.2 ROM 2
source: Android Central