PET bottles are readily available and inexpensive.  The standard thread for PET bottles for carbonated beverages between 0-3L in size is the PCO 1881 thread.  This thread type suits our carbonation caps that you can buy in plastic here or stainless here.
 
Historically you would only be able to place a black or grey ball lock disconnect onto the fittings one at a time.  This tee piece now enables you to attach both a liquid and gas disconnect to the bottle at the same time so you can essentially use a common PET bottle as a keg. 
 
NOTE: The PET soft drink bottles and carbonation caps sold separately.