theyoungone

Napa has EVERY size bolt you could possibly imagine for car use. You can even bring a broken bolt and use their display of nuts/bolts on a plactic stand and it tells you the thread pitch and bolt diameter size. Same thing for nuts. The bolts you can choose by 1mm increments I think.
BTW, They have a good selection of metric bolts/nuts as well.

BlinD

All you guys must have cool Napas. Ours is GHETTO like 5 feet wide and dont got ----. I'm going to look for another one though, maybe can hit it up.

dubmonkey

one solution is to look for carriage bolts or any other bolt that is threaded all the way to the head. Cut them to length and use the double nut method to tighten. This way you can get the correct pitch/size and grade level.
(don't use threaded rod).

I'm not sure of your exact application, if you are threading to the adapter plate, then (I assume) made the holes and tapped them yourself, if you are trying to thread to the turbo, perhaps it would be easier to drill out the holes and use a nut/bolt combination. (or retap the holes to a larger bolt/stud/etc that you CAN find)
Some napa places can't figure out parts unless you ask them for specific applications try asking them for exhaust manifold studs.

good luck