I have a tecumseh 7 horse (Snow King) motor that has a blown head gasket. Model number is HSK 70. Where do I find a part number to order replacement gasket?

That does not sound like a complete Tecumseh model number.
You have several options... Your first and probably best option would be your local small engine repair shop. They may even have the parts in stock, if not in stock will likely have it in a few days. Or you can look online at places like outdoordistributors.
No matter where you get the part from you will need a complete model and spec and maybe a serial number ( off of the motor itself). Also be aware that since Tecumseh went out of business a few years ago that certain parts are becoming increasingly hard to find.