Is there any substitute for saltpeter / sodium nitrate in corned beef brine?
Saltpeter is potassium nitrate, which does not directly cure meats. Bacteria convert nitrate into nitrite, which is the real preservative. Saltpeter can be replaced by a smaller amount of nitrite to get the same curing effect (most commercial cured meats do this), though a prolonged cure that converts nitrate into nitrite can develop more flavor.

Tender Quick is not a direct substitute because it contains mostly salt. I've heard that you can replace the salt in your recipe with Tender Quick, and drop the saltpeter, and have a success. You would have better luck finding a recipe that was meant to use Tender Quick, though.

It is definitely possible to buy (food-grade!) saltpeter. I would check online, or at specialty stores. It's a little more difficult than picking it up at your local grocery store, of course.

(Chemistry lesson, courtesy of McGee: nitrate (NO3) is converted to nitrite (NO2), which then reacts to form nitric oxide (NO), which bonds to myoglobin in the meat, which turns it pink and prevents oxidation.  Nitric oxide is also present in smoke, which gives that "pink ring" around the outside of smoked meats.)