I am not a plumber, but I have seen the local plumbing supply houses carry PEX pipes. You can look in your local Yellow Pages for plumbing supply houses in your area.
Each manufacturer also has adifferent system for connections and fixtures which you will need. In addition, the pipes used for heating loops must also have an oxygen barrier.
Also, some of the manufacturers will not guarantee their product unless it is installed by a factory trained and authorized installer. You can get a list of the company distributors/installers from their web sites listed below:
Wirsbo: http://www.wirsbo.com
Vanguard: http://www.vanguardpipe.com
Ipex: http://www.ipexinc.com/kitec (they make Kitec (a sandwich of PEX, Aluminum and PEX: PEX-AL-PEX)
Other: http://www.radiantmax.com (they sell Kitec tubing online)
Good luck.