Tricky. I only know one repro 12 gauge, a limited edition over under percussion which is becoming sought after, so you are probably talking antique. Most 12 gauge doubles fall in to the category of 'Belgian clunker made for the American market'. I will shoot most anything muzzle loading but I've never wanted to shoot one of those.

In England twelve gauge didn't become popular until live pigeon shooting and was soon superceded by cartridge so they aren't common, they are expensive.

You might get lucky. If you lowered your sights to a 16 gauge, Birmingham gun you wouldn't need so much luck. Singles are common and real cheap.

For clays, 12g rules. I gave up looking for an English double and bought a pristine Austrian 13g. Quality gun and a snip at £1350. I grabbed it so fast there were some fingers still attached to it