You could get a manual focus Tokina AT-X SD or Tamron SP 300/2.8 lens for $800 US, or a bit less. You might find a used Nikkor 300/2.8 *ED AIS in rough cosmetic condition for about $800. I had the very similar Nikkor 400/3.5 *ED AIS, and I was not impressed. You can find out more about Nikon lenses at http://www.naturfotograf.com/lens_surv.html#rating and http://www.donferrario.com/ruether/slemn.html.
I now use a Mamiya M645 A 300/2.8 APO that is truly excellent, but it cost me about two times your budget.
If you don't really need f/2.8, then you could get a Canon EF 300/4L (non-IS) for about $800, and the IS version for about $950. Cogitech was very happy about the FD 300/4L that he had converted to EOS, but I don't recall people talking about a converted f/2.8 recently. The SMC Pentax-M* and -A* 300/4 have absolutely excellent reputations, and they're very short in length compared with other 300's, but I don't think they'll be better than the EF 300/4L (IS or non-IS).
A lot of 300/2.8 lenses are in the Lens Price Database, Lens$db, at my web site.
I have both Tamron adaptall and Nikon. Both are very good. The Tamron is better made while Nikon has lower CA but vignette at wide open. Tamron gives you cleaner/a tad of green while Nikon is darker and a tad of yellow
