#3 - eBay
#2 - Hobby Lobby - Vacor mixed net bags are cheap and I've had great luck finding some beauties.
#1 - My favorite place, by far, to buy marbles is right here at work. I work in a medical facility where our patient population is comprised of many elderly people who have held on to not only their childhood marbles, but those of their parents. I have purchased quite a few, but have been gifted even more. If I were to come across some CAC flames, etc. I would be honest and let them know they have something really special. But so far that hasn't happened. My latest gift was a nice lot of Vitro Hybrid Cats, some of which are real killers.