I got mine through either NAPA or Ford, but as I recall it was part of the upper gasket set not by itself. Let me dig around and see if I can find it by itself somewhere.
Are you talking about #9E436 (7.3L; DI Turbo; Seal-Turbo compressor outlet(mounts between 6K682 & 6K889))? It is $5.54 at fordparts.com. If you go to FordParts.com, either put in your VIN number or your vehicle year/make/model. That'll take you to the catalog. Select Engine under Category, which will pop up a question about which engine you want. Select the 7.3 V8. Under Sub-category, select Gaskets and Sealing Systems. I found it on page two, titled "Intake Manifold Cover Gasket".