Does American Muscle have mil eliminators for 05+ GT. If not, where can I find them?

As far as I know, the only way to disable the rear O2 sensors on a 2005+ is with a tune. I know a few people were working on 2005+ MILs, but had not succeeded yet. If anyone has successfully made a 2005+ MIL eliminator I have not heard of it.

so to put x pipes on the new mustangs you have to have a tune?

As of right now, yes.