Does anyone know where I can find a simple circuit or cheap IC that can convert an SPDIF stream into analog audio?

optical or coax?

Check www.partsexpress.com and search for converters or digital converters

Coax.
I'm designing an amplifier just as a personal project, want a feature to accept SPDIF in, but I need some information and help on converting the signal to analog.

Rod Elliot's Project 85 sounds like it is just what you are looking for. I haven't built one myself - but it looks like it will do what you want. Unfortunately, he doesn't sell boards for this like he does for many of his other projects.

There are some good articles on this page which you might be interested in if you are designing an amp. There are also some good amplifier designs. I built one of his amps and I am very impressed.