They show up on E Bay once in a while, also look for 1831PX (Sealed power) the X means forged, 

I don't know how bad you need it or what your willing to pay but I would contact Ross Pistons in I believe El Segundo Ca. Go to their website an see they make custom pistons for flatheads and FE's plus all other  Fords even though they don't show a Y-Block. They've made me custom forged pistons for my race and street GMC's. If you send them one I'm sure they could copy with the same weight from one of their other forging. That what I did for my sons engine gave them one to copy and they made 6.  You need to make the ring package is the same if you already have the set..
I just bought a single Silvolite CAST off E-Bay for a std 312 and honed out a 292+.040 and extra .010" and was lucky it was 3 grams light and works great. Even bought std rings too.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

A GM 4.8 (LS) piston will fit with a 6.7" rod and a little work, this gets you a modern ring pack and cheap pistons.