If you are looking for something cheap, try Magnaflow mufflers. It will have to be a custom install job at your local muffler shop, but they are relatively cheap and good quality..
<span style='font-family:tahoma'><span style='font-size:8pt;line-height:100%'><span style='color:red'>'98</span> 540i/6 Biarritz Blue/Beige Beast
"The best **** car I've ever owned"
<span style='color:blue'>Euro Xenon AE's
Celis Clear Rear Tails
Rogue Engineering Octane SSK w/ WSR
Rogue Engineering Transmission Mounts
Royal Purple Synchromax Transmission Fluid
GruppeM Ram Filter Charger</span></span></span>