can anyone tell me where I could find a kit for a huge cannon, about twice or three times the size of the imperial cannon?

I want to make a turkish style greatcannon, and cant find any relevant miniatures.

Well, if you wanna go chaos, try the hellfire cannon.

For empire check out a 1:32 model kit, from revell or airfix or some other brand. They might have a kit of a Turkish great cannon or you can buy a scale that's bigger than GW's cannon. (GW is about 1:45, just about).

A larger scale model kit is a possibility, but I think a Turkish great cannon won't be the most common type of model kit out there. If you're any good at modelling you could scratch build one. Great cannons tended to be fairly simple in appearance, as I recall. I seem to picture them in my mind as looking simply like a big, banded metal tube on a base, so if you can find an appropriately sized tube to use plus some plasticard or something you should be able to put one together.