Usually, people would buy either:

a) A DVD player with 5.1 decoding built-in, or,

b) An amp/receiver with 5.1 decoding built-in.

Since both of the above items are terribly common, the need for a separate external decoder is so rare that no one could make money selling them.

At least not now they can't. I remember when DVD players first came out, there were a few external 5.1 decoders on the market to help people "transition", but I haven't seen one in literally years. There's just no market for them.

For now, just use the stereo outputs from your DVD player into the stereo inputs on your amp/receiver. I would assume that your receiver has regular Dolby Surround decoding built-in? That will work fine, you will get surround sound from all of your movies. It just won't be six discrete channels, it'll be four matrixed channels, which is still pretty good.

I have to admit that I'm not 5.1-capable in my household yet, I'm still running regular Dolby Surround and I'm quite happy. Just tonight I was watching Jay and Silent Bob Strike Back and thinking to myself how good the system sounded and how well-balanced it was.

Eventually, you'll get a new amp or a new DVD player and you'll be able to get 5.1 decoding in whichever one of those items you get. But for now, don't sweat it.