Ebay. That's by far the best place to spot really good gear for a small fraction of the sticker price.
The usual suspects for brands, personal favorite: Panasonic Minas series drivers + associated servos. Cheap, super reliable and available in just about every size that you could possibly want.
I've looked into this a few times and it always seemed like there is a huge abundance of servo motors, but the market for servo amplifiers seems to be tiny in comparison. I always suspected this is because the electronics of decommissioned machines are just scrapped wholesale. However, I am also wondering what people are doing with all these servo motors without matching cabling and amplifiers.
Good point, amps are a bit harder to get by than motors, but then again, there are plenty of them on offer. I only paid full price once for a set of servo amps and that was because I wanted to have a particular type for a very special set of motors (pancake servos).
How do you deal with the proprietary connectors on the motors? Replace them or only go for motors that come with stub cables? Proper replacement plugs probably cost about as much as a used motor... and most seem to be sold without cables/plugs. (Unfortunately it is hard to search for this stuff, because everything is "polluted" with model making servos)
The usual suspects for brands, personal favorite: Panasonic Minas series drivers + associated servos. Cheap, super reliable and available in just about every size that you could possibly want.