its not so much special equipment, stainless can be done by any of the same means as Mild steel (mig, tig, even stick weld if your desperate!) the important thing is the correct wire is used..
we generally use 316L (i think) at work...
most place like kwik fit or other exhaust places (certainly round this way) will just have a MIG unit armed with mild steel wire, now while this will weld stainless and work, the weld wont be as strong as it could be and introducing mild to stinless will allow it to rust..
probably best trying engineering places, we`d only charge a tenner or so to whip round an exhaust providing it was cleaned and removed