Try going to a pull and save. You can probably order one from Summit Racing for about your price range.

Never mind. Looks like a whole unit. Expensive. http://www.summitracing.com/search/Year/2004/Make/CHEVROLET/Model/BLAZER/Department/Exhaust/Part-Type/Catalytic-Converters/?Ns=Rank|Asc

Otherwise, give Stadium Auto Parts a call, I'll bet they have one for less.

EDIT: Get off my koolaid gnihcraes!