The reason you can’t find free SAT subject practice tests online is because there aren’t any. Your best bet it to go to a local library and find the College Board’s SAT2 book for math 1C and 2C (single book for both). Take the practice tests there. Or just buy the book on collegeboard.com or amazon.com (if you don’t write in it, you can resell it on half.com when you’re through). The best practice is to do practice tests by the people who actually write the test. Good luck!