Here are two internet sites that deal in engine cores. The third deals in remanufactured engines, but had offered just to sell me a magnafluxed block.
www.coremarket.com
www.enginecore.com
www.remanufactured-engines.com (800-492-5784)
Here is a site that lists casting numbers for heads, blocks, and cranks, for almost any engine, from almost any manufacturer:
www.Fred-Jones.com
To answer an earlier question, I had my short block built at Wayne's Engines in Riverside, CA (909-684-2222) (Burt). I told him what casting number I wanted, and he found one at an engine core warehouse in Las Vegas (I lost their phone number). So he built an engine exactly the way I wanted it.
I hope this helps.