You can get the O2 sensors at AutoZone. I'm sure you could rebaffled your airbox, but you shouldn't have to. It does give you about 5 HP if you debaffle and put a green filter in. O2 sensors are more expensive if it's after the CAT.
<span style='font-size:8pt;line-height:100%'><span style='color:blue'>1998 BMW 323is Coupe, Black
18" M3 Hyper Silver Rims
2 12" Rockford Fosgate Subs; Sealed
1.0 Farad Capacitor
800 Watt 2-Way Rockford Fosgate Amplifier</span></span>