This is going to sound sarcastic (but I'm really being sincere) but try a university library.
Even some small, two-year colleges, have libraries that have good textbooks on mathematics (and number theory).
If you want specifics, I'll get back to you on that one.
For a general approach, try Miller and Hornsby's Mathematical Ideas. It's designed primarily for education majors but I think it's a great book for the basics of a lot of math fundamentals (including number theory).