A reputable seller on ebay, possibly Borg Warner them selves. If its an old style the exact wheel may not be easy to find. Also, turbos usually have to be rebuilt after rebuilds, consider having someone rebuild and upgrade the wheel for you. Best guess is to call borg warner themselves and ask.

Old thread, but there is no billet S256-ET, I don't think. BEP would make a S256 with a billet S259 wheel on demand apparently, but that would just be an S259.

BorgWarner makes the S200SX with billet 57mm FMW wheel, aka AirWerks 7670, not to be confused with the EFR 7670 (the names are ridiculous, I know). The S200SX FMW uses the EFR 7670 compressor wheel on the S200SX-56 turbine.

A BorgWarner S200SX-56 uses the same compressor inducer diameter as the S256, otherwise there are few similarities. The 76/65 turbine of the old BorgWarner S256 is outdated, the 70/61 S200SX is much improved.

So, if you want a billet "S256", the latest version is the S200SX-57 FMW. Available here: http://www.full-race.com/store/turbos/borgwarner-airwerks/borgwarner-s200sx-fmw-57mm-turbo.html