Tamron do a Tamron AF 200-500mm f/5-6.3 Di LD IF SP (which has the 'SP' bit, the important bit on any Tamron lens name) but other than that your looking at Canon's big prime lenses, and "cheap" with them is a very relative term
If you combine the 100-400 with one of the teleconverters (1.4x is optically better than the 2x one) then you would have a big increase in range. Canon, Tamron and Kenko make them, but they won't work with all lenses, so make sure before you buy (and the 2x one will probably stop your AF working)