I'm confused as to why you think the evaporator is bad. If the car isn't cooling like it used to - you need to have the cause diagnosed. It could be a leak somewhere else, a compressor going bad, or numerous other things - ALL of which will cost you less than replacing the evaporator.
The ACM evaporator is supposed to be the best aftermarket replacement. I've been researching this topic for 2 weeks now, after the evaporator in my 1995 E420 crapped out, and it seems to be the consesus of everyone that I've asked.
For cheap parts - try Fastlane (click at the top of the page) or www.importedcarparts.com
- I've used them before and their parts are really the cheapest - fast shipping too.