There's not really any website I've seen about them exclusively. They are just basically an expansion chamber performance pipe. They are about 80 to 100 bucks depending on where you shop. They are only made for Puch and Tomos, but people have modified them to fit on other bikes. I've got one on my bike (1977 Puch Newport) and it adds about 5-7 mph top end. It's not too bad of an exhaust, but Polini and Leo Vinci make better exhaust systems. You can get Biturbo exhausts at
www.themopedjunkyard.com (this is where I go for most of my parts)
www.mopedsonline.com (they also sell polini and leo vinci)
www.mopedwarehouse.com (so I hear, but they never respond to my parts requests)
and there's a few others, probably. Look around.