my joists are 12" OC. Is there a brace that would fit in this space?

While it will be disavowed by the manufacturer, this brace can be easily modified for shorter spans. I live in a 120 year old house. The joists for the second floor are 12" o.c., full dimension 2" x 14" lumber. I have 10" between the sides of the joists. Remove the screw on the end of the pipe securing the rotating bracket. You can then pop off the bracket with little trouble. Cut the pipe to your desired length ensuring the cut is square. A pipe cutter is recommended. Remove all burrs. Drill a 9/64" hole in from the end 7/16". Replace bracket and secure with the original screw. Cut the long bolt to the desired length. Chamfer threaded end and reassemble.

Unfortunately, we do not offer a brace for joists with less than 14 inch spacing however we do offer a 4 in. round ceiling box, model #294, that features a brace that only needs to be secured to one joist. It can be used in new or old construction, can support a ceiling fan up to 70 lbs or a fixture up to 70 lbs in new construction applications or up to 150 lbs in old construction applications.