Sounds like you have CA emmissions, as you mention OEM wiht cats on those downpipes. If so, $600+ per for them OEM. I did see a source once for aftermarket, can't recall where. They were something like $250.

Remember that aftermarket cats will not last anywhere near as long as OEM. Just saying...

You say no emissions in MI. Based on that, try getting the non-CA downpipes. Not sure if you need a different Y as well. You'll have to deal with the second bank of O2 sensors in the process. MBrannon can point you towards some ECU-fakers that plug in for something like $50 each. The 3S crowd uses them when they modify their exhausts.