Just a heads up, some shops may not help you out. The Y-pipe is before the CAT on our cars, so it's "illegal" to modify anything that is cat or before, per CA law. (Even though the y-pipe mod, is very simple and easy to do with the right tools)
Now i got it done in a Northern California shop, but i no longer have a conntection there.
If the shop you contact is cool, they may do it after hours as a "project" but may ask you to pay in cash so theres no paper trail.
Just don't be surprised if a couple shops you contact turn you down at first.
Good Luck,
LUpe