As i see it, you have got three options
Get a soundcard that provides you with 5.1 out put (i belive that most of them provide this solution not, and i know that Soundblaster has a verison). The only problem is which connectors is on the card or its aux output box (front thingy).
You can buy a decoder only (it doesnt amplify) but it will take digital 5.1 and make it analog for the normal amps you have. I believe this is the cheapest and easiest solution to your problem
or you can as you suggest buy a cheap 5.1 amplifier and use it to divide the signal for the remaining amps.
i cant recommend any products or links since its been a while since i purchased my equipment and i stopped watching price / quality and preoduct reviews.