One of the best sources for a cappella is this this site, Primarily A Cappella, which sells CDs of dozens and dozens of a cappella group — contemporary, jazz, gospel, barber shop, collegiate, world, doo wop, vintage harmony, etc.
There’s even an onsite a cappella audio stream.
For the record, I have absolutely no financial ties to the site other than as a longtime customer.
For really good contemporary stuff I second the nomination upthread of the BOCA (Best of College A Capella) annual compilations, which started in 1999.
For non-college contemporary groups, AC Rock, The Coats (they were The Trenchcoats before Columbine), Rockapella, 17th Avenue All-Stars, the inimitable Bobs, Throat Culture, the Nylons and the oh-so-smooth Blenders are all phenomenal, but there are a huge number of talented groups out there.
If you’re anywhere near one of the regional competitions for the Harmony Sweepstakes, by all means take it in. Mrs. HTB and I have been to the Pacific Northwest competition twice in Olympia, and enjoyed it hugely, though it’s a four hour drive each way and an overnight stay.
It’s particularly delightful for those of us who cant carry a tune with a forklift and a dumptruck.
Happy listening
Hometownboy