Actually you can use a Flowmaster Camaro muffler for a custom side exhaust setup. But to be honest with you get the Spintech Side exhaust kit. Sounds pretty much the same, maybe just a little louder.
Not familiar with the Spintech set up, although they look nice. My SVO side exhaust came with Borla mufflers. I would think they would be available directly from Borla, and you couldn't do any better on quality.