SUBTYPER

I HAVE A 98 IMpreza WRX STi Ver 4 and have had fitted a Magnex downpipe. But when the garage fitted the pipe. the actual lambda sensor is rattling about inside the nu. which I take is causing a air leak and also wrong lambda readings. i have been told that i need to replace the lambda sensor with a new one but no one can tell me which Lambda sensor to buy or what part code to order. HElp.....