I was wondering where I could get a aftermarket Y pipe that will bolt on to stock manifolds.

 

Where do you want this y-pipe to go, before or after the cat(s)? If after, you need a 90 degree elbow in 2.5" diameter (I recommend mandrel bending). Then, a dual 2.5" input y-pipe. Flowmaster and Magnaflow both have good designs. dual 2.5" in to 3" out seems like the best to me.

You could get CARM10706 and CARM10778 for example. Realize, these are both aluminized.