where can i get a quad decoder that slides into the bottom of my receiver, i've checked on ebay, is there a parts place that might have one.........thanks

I do not know if any constant source for quad decoders. In my experience, ebay or audiogon is pretty much your only option. Every once in a while one pops up on my local Craigslist. Perhaps someone here might have one that they are willing to part with.

By the way, the specific part you are looking for is either the SQA-1, SQA-2 or SQA-3, any of these three will work for your Marantz 4240. Keep checking ebay, I am sure one will pop up eventually. Be prepared to spend somewhere north of $100.