Ebay, since Favorite/Piqua is seldom found in California. Lots of Wagner here, less Griswold, still less Lodge (old, raised numbers) so I have to do e-bay to get my fix, even though the shipping is killer.
Other places, roadtrips to my parents in Idaho that cut through Nevada. Bought some beautiful pieces in Reno recently, including another Griswold 380 camp oven for about $100 to go with the one I bought for $60 a month earlier in Rexburg, Idaho.
Alameda and Alemany flea markets (I always seem to arrive after Steve has gone through though. But the gentleman that he is, I always find something interesting left over to buy...)
And my secret fishing hole is a garage here in San Francisco where a slightly off kilter older lady runs a perpetual garage sale. She goes to others garage sales, argues them down, then brings the cast iron back to her place and sells it to me for anywhere from a minimum of $3 to a maximum of $5 per pan. Its like having my own cast iron valet service...