From what I have seen, SLS is the only company that still makes aftermarket midpipes for our cars. Other than that it is a custom fab job, but I have read from other members that you can buy a high flow cat and some piping, and a shop will do it cheaper than even the catless SLS midpipe.
I guess also if you consider the MSP midpipe an option as it isnt stock, then that is an option?