Thanks for your replys, sigma and tamron are the two i've seen on google, but like you say, they are hard to find for sale. i will have to bide my time and keep browsing or make do with a 500mm, plenty of them for sale.
Sigma and Tamron both do zooms that reach 600mm at around £1000 give or take £100 - significantly cheaper than a Sony lens. They generally receive good reviews from wildlife photographers.
Used, these can be picked up in good- excellent condition between £500 -£800, but don't come up very often, I guess people who have invested in this level of lens are less likely to up grade.
%00mm is much easier to find 2nd hand and new.
In Britain we have a number of excellent camera shops that sometimes get used BIG lenses in, or MPB who specialise in used equipment - I'm afraid it's a case of keep on browsing.
Personally, I use a Sigma 170-500mm APO if I specifically go looking for wildlife, or my Minolta 500mm mirror lens if I'm out in the countryside and might take landscapes or wildlife. (The mirror lens has a fixed aperture of f8 - but is my lightest lens, whereas my 170-500mm is my heaviest lens)
Have you thought of a 1.4x or 2x converter?
Really LONG A-mount (or E-mount) lenses can be hard to find and will be expensive, large and heavy -- whether zoom or fixed.
For really long work, you might want to look at non-A-mount lenses. They are, of course, non-auto-focusing, but are easy to find and much less expensive. They can be found in refractive or reflexive models up to 2000mm -- and you will need an adapter.
I have a Honeywell 1250mm f10 that I got for $100.