If you can afford it www.currieenterprises.com has them new. Also an early rear from '64 to'67 is 1 inch narrower than '68 to'72 but will still bolt up. This helps with getting a wider tire size. I have a low mileage '66 12 bolt posi that would take a lot of convincing for me to get rid of.
I'm trying to find a 12-bolt for my car. When I find one, I'll make you a heck of a deal on my 10-bolt. I'm not too far from you either (Dallas). Problem is, I'm not having much luck yet. I'd like to find a complete posi rear (with a 3.36 or 3.55 gear) for less than a grand.