Not really something you can buy, most people who have one built it themselves. There are several threads here about building gunge tanks, advice on release mechanisms, etc.
The most important thing is safety. A tank full of gunge is easily heavy enough to kill someone if it falls on their head - it could break their neck. So in building any gunge tank, always consider the strength needed to carry the load, and always test the first time without anyone in it, just to be on the safe side.
I have designed my own but haven't built it. It is essentially wood construction. 4 4x4 post with 2x4 bracing. A deck for the tank to sit on made of 1/2 plywood with a hole in the middle. A large rubber tub with a shower drain drilled through the bottom and a toilet valve to release the flow.
Safety is the biggest factor. This is not something you can just setup in your bathroom. I'd have to use my garage.
A gunge tank is just a box with a sealed hole in the base, but the frame is the important thing.
My gunge tank is ok/basic (see my profile for construction pictures and a working video) but the frame is just a scaffolding tower bought off amazon. Costs about £270ish and can take over 300kg of weight but that's perfect and fits in my house so I can get messy indoors. Fortunately I have other uses for the scaffolding and a large enough shed to store it in!
There is a few bouncy castle hire companies which carry these for rent too. If you do buy one outright- I've seen them resell on eBay for over £500. You can expect some depreciation, but it'll keep well over half it's value.
Love you, too
