There are alternatives to the Canon 1.4x; Kenko and Sigma, for example, make 1.4x extenders. I don't have any experience of them but I'm sure somebody will be along soon who has.
2x extenders are a whole different story though, for two reasons. Firstly the impact on image quality is considerably greater than with a 1.4x. Secondly you won't be able to autofocus with that camera/lens combination.