It doens't get much more all-around-general than the MacDaddy of them all:
www.cabelas.com
Other mega-stores include LL Bean at
www.llbean.com, and Bass Pro at
www.bassproshops.com.
Getting a bit more personal, there's Dan Bailey's in Livingston:
www.dan-baley.com. I've also used Hook & Hackle, they have a good selection of tying kit
www.hookhack.com. For ready tied flies, Hills Discount Flies in Washington, the real one as in the state, not DC.
www.hillsdiscountflies.com.
When your friend lands in America, tell him to get in touch. I'm in the book.
Grouse