there are a number of 35mm clamp mount derailleurs out there. Almost everybody offers a few models in 35mm. If you already own a braze-on FD, or want to keep it a bit more universal, there are clamp-on braze-on FD mounts like these.

The last option if you're up to it is to source a bolt-on mount from a frame builder's supply, and drill the frame, and bolt it on with riv-nuts or other means. I'm not suggesting that you do this, but you could.