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.