so somehow my downpipe to midpipe nuts/bolts came undone while i was driving because im assuming the previous owner didnt put in the correct ones so i was wonder what combo is recommended to be used. single turbo fd3s with custom 3" DP and custom 3" midpipe (same location as stock) PFA

 

Not exactly sure about the material make up of exhaust bolts..
BUT here's a quick idea..(at 3:40 in the morning..heh,heh!)
Go to your Auto parts store and get some generic Exhaust Studs..with bolts and washers.
Since the parts are meant for high heat they should work just fine.

 

Any bolt that fits in the hole will work. A smear of plumbers silicone works fine on exhaust gaskets too iv found. And a bit on the bolts acts like a low strength threadlocker. Although high-temp RTV is better if you have it but neither is necessary.