I did some research on it awhile back, because I'd like to do the same. The one that most impressed me was this company:
They make what looks to be a simple, but high quality kit. Check out the videos and testimonials.
They don't require you to send your upper receiver to them for installation, as some other companies do. Most of Adams' are do-it-yourself installs, if I read them correctly.
Pretty pricey for what the parts actually are, but if anybody out there makes a low-priced piston unit, I haven't heard of it.
Personally, I don't have a great need for a piston unit; my Bushmaster is absolutely reliable, but it sure would be nice to get that gas vented out somewhere other than into my face when shooting into the wind!