You might want to check out this site to start:
http://techno.org/electronic-music-guide/Nice overview, and a bunch of his genre buttons have links to streaming radio of more-of-same.
Trance really peaked about ten years ago - there was some really great stuff but it was all on CD and I'm not sure how you'd go about finding it these days. So many tunes done by guys that just put out one or two songs then disappeared. There were some excellent compilation disks, all the Trance Europe Express series were very well put together, as well as a series called A Trip To Goa that went about six volumes, and those were all very good.
Off the top of my head, there were a few standout groups that lasted through the years - Eat Static, Astral Projection...tomorrow I can poke thru my itunes, I've got about 500 gigs of stuff I collected back before techno was called electronica.
But again, the best stuff was on the comp disks. Mostly just because the guys making it would just do a song or two then vanish to become a different "band".
Update to above, I just looked around the itunes store and there's a comp set called "
Take A Trip To Goa", this is NOT the same compilation(s), and I listened to a few tracks and they sound pretty inferior. Avoid!