^Probably one from a 325i, the 318s won't work. A cheap muffler shop could probably hook you up for about $40.
In regards to changing it, i think its relativly straight forward, you might want to hit the bolts with some wd40 or something else the day before to try and clean them up and make it easier to get them off. You probably would also want to do new hangers (also could be the cause of the sound), and possibly some new nuts and bolts.