problems with catalytic converter

for the last couple of weeks our 05 touareg has kinda been shakey, we knew it was time for a tune-up but hubby has been working crazy hours. Today it started squealing really loud everytime he would let off the gas pedal. We went to to Midas and they said that it was misfiring in Bay 1 1-6. They told us that only VW would be able to fix it, but I found a couple of sites that sell them. If anyone could reccomend a good site to get one from and what kind is best for the touareg I would greatly appreciate it. We just don't know what to do. Any suggestions are greatly appreciated