Thrift shops, though times being what they are the supply of wool clothes is dropping somewhat lately.

For new, look at outlet shops, I bought a really great Woolrich wool coat at an outlet shop on the way to an offsite for work. It was either a Woolrich shop or Burlington Coat Factory. I've seen some wool shirts at Eddie Bauer outlets too.

Locally Pendleton Woolen Mills in Oregon puts out nice wool clothing, some of their shirts are offered in Macys stores, I have 30 year old Pendleton shirts that look like new. Visting Pendleton was like a trip to Mecca. There are other woolen mills around the US still, Faribault (Faribo) Woolen Mills is a favorite and has an outlet in Fairibault MN (not so much on clothing, but great blankets).