You probably can't find one because the main cost is "everything else" besides the dolby digital decoder chip...

Off the top of my head, I know of two digital decoders, neither of which are too cost effective. The first is the Creative Extigy usb sound card, which also works as a standalone dolby digital decoder. The seccond is sold by Klipsch as an add-on to their promedia line of PC speakers. I have the extigy as a sound card for my laptop, and it's a neat hack, but USB and sound were never meant for each other. If you've got a laptop, it's the best you're going to get however.

Both of these are goign to run you $150 or so new. You'd probably be better off investing in a new DVD player or reciver, unless the ones you have a really top of the line or you feel like adding a USB jack of sorts to your home theater system.

Matthew