Craigslist is your friend there. Expect to spend $1000 on average, although "deals" can be had in the $700 - $800 range. Miatanet is pricey because of the nature of the posters. Look at buying an entire car just for the top because although that may run twice or more as much, you can usually resell the car, sans top, and recoup most of your investment.

Also consider a Snugtop, if the aftermarket equivalent rule is effective here. Specifically not allowed in SM trim, it may be legal for IT. Rules nerds?