While I don't have any first=hand knowledge of any lenses (per se) at that length, I have investigated the possibility of purchasing a scope and adapter ring set for my Nikon. If you investigate it, it's probably less expensive doing it that way - a land-based (bird-watching) telescope - you can get a decent one for around $400 or so, then get the adapter ring, and you'll have an 800mm-1m or more lens. Not the fastest aperture to be sure, but you'd have the reach.
Nikon | Sport Optics | Digiscoping System