As the title says, I need to get the slipfitter clamp to stop wiggling around cause it makes a lot of noise. What torque wrench should I buy?

If you can remove the bolt, you should be able to go to your local hardware store which should have some thread size testers, well, if its anything like Bunnings here.

The bolts don't move.

Just about all cobraheads utilize a 3/8" bolt, which has a 9/16" hex head. This is true for the OVF as well.

Yeah most cobraheads use a 3/8" bolt, I think 3/8-16 for the slipfitter bolts. I'd get a good breaker bar or better yet a impact wrench for loosening them, although some penetrating oil such as PB Blaster on the threads will also help.

A torque wrench is useful for tightening them to the arm so they're not over torqued, if you have the instructions I think most modern cobraheads have a recommended torque range for the slipfitter bolts.