I'm interested in after market seats for my Legacy wagon that have more substantial hip and shoulder bolsters than the stock seats. Are they out there, and what about seat bases/rails, etc.?

If you're talking about racing seats, they will work. You could also try to get a pair of front seats from the STi. The sliders and everything are the same between Imprezas and Legacys, I know. So, if a shop does you like they did me (before i knew that) and tells you that they don't make sliders for the Legacy, you can fill them in on that little detail.

Thank you John --
I forgot to mention that my model year is 1997 -- are sliders the same throughout the years?

As far as I know they are the same. I'm not absolutely sure, but most parts are pretty interchangeable in subarus.