There are loads of options out there if your replacing exhaust parts. The Bypass pipe is welded into the existing system. Its 2 1" diameter pipes the idea is that it lets some gas out the system without going through the rear silencer. So it increases noise a little. It wont help performance but i don't think the aftermarket exhausts do much any way?
The quad system although not expensive for me to make. About £150 would cover costs and my time. Its the fitting that is labor intensive and time consuming. Same as the bypass pipes really I can let you have a set for £20 probably but its the fitting of them.