340 .040 over pistons for Zero Deck...Where to find?

A few thoughts -
1. If the rust is low in the bore it's not critical. it may not ever even get near the rings. So you might not need to get anything funky unless you wanted to.
2. There are various ways to skin the quench cat... Most pistons are designed to come up proud of the deck when installed in a deck machined at blueprint height. But most decks are significantly taller than blueprint height. So unless you are instructing the shop to blueprint the deck height, or giving them a deck height to cut it to, then you really don't know where things are. If you are serious about building for quench, you need to have the deck square decked either to blueprint or to a height that works for your rotating assembly. Simply standard decking for flatness is not enough accuracy.
3. If the pistons you ultimately decide to use are proud, you can always run a head gasket that will set the quench you want. I only use FelPro 1008 with aluminum small block heads becuase of the pre-flattened fire ring. That's a .039 compressed gasket. IIRC 9.600 is blueprint. If you have the deck square decked to 9.6 then the pistons designed for +.018 above deck will come out around .016-.020 above due to stroke and rod variences. So you will have to have the pistons milled down by .015 to get the quench to .035 or so. If your deck is taller (and the shop that can square deck can also tell you this easilly) then have it square decked to 9.615.