So far most of the cast iron around here has come from yard sales, but so far I haven't found a cast iron bread pan. Online there seems to be three major ones to choose from, Lodge, Sante and Old Mountain. I've had some new Lodge cast iron pieces lately and they've been really rough and unseasoned. Had to use a grinder on them to get them smooth enough to use, so that would be my last choice of the three. The Sante seems to be a nice shape for making slicing loaves since the sides and ends are more vertical. The Old Mountain pan seems pretty slanted which does let the bread come out easily, but isn't so good when it comes time to slice it. So, my vote would be for the Sante pans. Just wish they'd show up at a yard sale!
King Arthur flour had one for awhile, but I don't think they sell them anymore. Neither does Lehman's Hardware. Does Wagner make one? They seem to have smoother cast iron than Lodge. Hmm, Wagner and Griswold were bought out by the American Culinary company and they don't seem to make bread pans. Well, I guess online is your only option? Unless yard sales are excellent in your area.