I'd also endorse KEH as a great used lens source. As mentioned, Sony lenses can be found not only under 'Sony Digital' but also under Minolta Maxxum. There are 4 categories - brand zoom and non-brand zoom, and brand fixed and non-brand fixed. Typically, the prime macros like you are searching for would be under Maxxum non-brand fixed.
I can't speak for the Sigma 105mm or the Sony 100 - I was happy enough just getting the Tamron 90mm at an excellent price. I have had good experience with Tamron lenses and honestly doubt there's a significant difference between any of these lenses - the HSM lenses might be slightly faster to focus, or quieter, but those tend not to be issues for me when shooting macro as I'm rarely in a rush to shoot - so 14 milliseconds less focus time won't have any noticeable effect on me!