Koeze, and other modelers,
The replacement bladders are used in the arm cuff of a sphygmomanometer(blood pressure device). The kind to use for model use is the Neoprene bladder. The ones that come in the blood pressure monitors are now thin plastic, and do not have the strength nor the elasticity needed for our purposes. Internet sources are , AllHeart.com , or SafetyStore.
Also, the source for the small air compressor(pump) is , Goldmine-elec.com. Search for air pumps.
Hope this helps you and others who may be interested in RCABS. (Recirculating Compressed Air Ballast System)
RCABS- It's easy to control your own bladder!