Best chance to find a free tank model would either be here (Click on the "Free Stuff" button at the top of the page) or on ShareCG (www.sharecg.com).

The only problem is that you're unlikely to find a high quality model of a modern tank (like the American M1A2 or the Russian T-90)...and if you do...its probably not textured.

By the way...not trying to be smart or anything...the Stryker is an American armored vehicle (based off of the Canadian LAV III and the Swiss Mowag Piranha). Sometimes little things like alternate names help when you're searching for something specific. Good luck.