yeah they are extrememly expensive and only made by one company, panart. lots of people have made homemade ones out of propane tanks and such (youtube it)... but they doesnt even come close, it could still be a fun little project though.
i met a Girl (hippy) who was shredding on one of these at an art gallery but it was on a stand, so no slapping on the bottom.it was pretty crazy the shit she was playing for sure....and it smelled like reefer in her corner obvs.