Any auto-parts store or wrecking yard if you really want low cost.
I have the 2.5", TTI, x-pipe system on my car along with stock manifolds.
It fits and functions well. The pipes to the manifolds and intermediate pipes that connect to the x-pipe were a bit of a pain to install but not a big deal.
I have ZERO extra room to remove my 4-speed with the system installed (I can get the trans in and out but it is very tight).