First of all, you probably do not want to use a bellows with your Canon EF lenses. The reason for this is simply that the lenses are all-electronic so you cannot change the aperture once the lens is on the bellows... unless the bellows are electronically compatible with the EF mount and I seriously doubt that such an animal exists.

However: The nice thing about Canons is that the mount flange distance is smaller than most other brands, so adapters to other mounts are easily available. My suggestion would be to get a Nikon adapter, for example, with a Nikon-compatible bellows of some kind (I say Nikon because it was the go-to brand in the seventies and eighties for most pro photograpers, so this kind of very-special-purpose gizmo is more probably available for Nikon than for others) and then hang an old, all-manual Nikon lens off the front of it. Such lenses are available for little money, and you will be able to change the all-important aperture manually at need.

As for why such bellows are not readily available, I'd suggest that since Canon has had a trio of very good EF-mount tilt-shift lenses available for ages now they cover the needs of most of the potential bellows users, and so remove the market for one.