eBay has listings for 'em, but they're usually the offset inlet/center outlet type, which won't usually hook right up to standard tailpipes without some modifications.
Dunno of anywhere else, other than directly from Spintech. They sound awesome, but they're too friggin' pricey for what they are...
man, i paid $105 shipped for my pair of spintechs 5 years ago...call some local muffler shops around you and ask if they can order them. i know a couple shops around here can.
Prostreets are the loudest. I have had the Sportsman split case on my car before with the side exits, they sound pretty nasty without cats. The Pro streets are nasty with or without cats. Here is a sound clip of my car. It's not the best quality but you will get the idea (e-cam, shoty headers, off road h pipe):