Hi - In addition to the umberella online second hand book sites that have "antique, rare historical and collectable books" (Abe Books and Alibris selling for S/H book shops) you can now buy online from OXFAM who list 60,000 books (inc 7670 antiquarian and 8674 historical books), all at most realistic prices.

I have just discovered this site today and already found four "must have" genealogy related books.