There are 1.4x and 2.0x extenders. I wouldn't touch the 2.0x because of the image quality that it robs.
The 1.4x would get me out to 560mm but it costs you an f-stop, so I'd be at 560mm f/8 and yea, it also robs *some* image quality...but not so much as to be useless.
The problem here is that neither my 5Dmk2 nor 60D have an autofocus system capable of operating at that aperture. My film 1V does just fine. If I wanted digital to operate, I'd need to invest in a newer and possibly higher tier camera.
If you're used to shooting old school cameras, then you probably remember the focusing aid on the ground glass...either a microprism, split screen, or both. Once things went autofocus, these aids wen't out the window. Manually focusing at 560mm, with no aid, and at f/8's worth of light is...really, really difficult.
So, yea, there are some extenders...but they add quite a bit of challenge to use.
F8 - phew boy!
The manual lens focus aid looked something like this: