I was looking to get a 3" downpipe for my L36. Where can I get one? Would a downpipe made for a grand prix fit my bonneville?

They have different exhaust manifolds. Any exhaust shop worth it'* pipe should be able to fab something up for you. However, you don't have the U bend, so it won't gain you as much as say, a High flow cat, or muffler.

if i were to purchase a set of grand prix exhaust manifolds, would it be possible to use one of their downpipes? Also, would they even bolt up to the bonneville heads?

Thanks.

Same heads, but i don't think you'd want to, no gains, the bonneville exhaust and intake setups are less restrictive, which is why they're rated 5HP more from the factory.

Best bet would be to get headers.

So you're saying there is no baffling in the downpipe?