Hi,
I have the 70 - 200 f2.8L non-IS lens. I got the non-IS as it has less glass in it and so the image is a bit brighter and the image quality is a bit better. Also, for astro use, the IS is off anyway, so it would be an extra $800 for something that I would not use that much.
Anyway, both lenses are really very, very good. I doubt if you could detect any difference between them and a fixed focal length lens of the same aperture - eg the 200 f2.8L
I got my lens from
http://centre.net.au/Canon_0007T3.html They have one in stock (according to their webpage). Otherwise they can be a bit slow in getting stuff - but they are generally very cheap.