Clare, I am impressed. Good for you. That is really neat. But some questions first:
1. How are you supporting the camera now, and does that support not work properly?
2. Indoors or outdoors?
3. How mobile does your kit need to be?
A couple of ideas:
1. The film industry has always had sturdy tripods for heavy equipment. In Australia, I do not know where used cinematography equipment is sold or traded.
2. Decades ago, companies like Deardorff and B&J made big wooden studio supports with casters. Finding a used one: difficult. Some people have them in their parlors as decoration.
3. Wood tripods: the large version of the Zone VI tripod from the 1980s, one of the larger Berlebach units (formerly from East Germany), or the larger Ries (made in USA) might work. Cost: probably high.
4. A used surveyor's tripod. You would need to modify the top to accommodate a camera rather then a total station or other surveying instrument. Cost: variable, possibly reasonable.