I'm not in the UK, so I can't comment on the availability there. However in my locality there were a couple of options.
Specialist office furniture stores that professionally refurbish - these were done to a very high standard, basically looked new, but the models were older, and the prices far to high imo.
Private sellers - easily the best prices, but very varied age and condition. The problem here is you have to drive around to try them out.
Finally, I found a couple of small time guys who refurbish these on the side, like a cottage industry. They generally advertise on local auctions sites, and you can pick them out due to the number of ads they list. Seems they get the chairs from liquidation auctions etc. These generally had a few options to look at, and prices were closer to reality. One lad had a couple of two-year old unused fully loaded Aerons still in their original boxes for £650 each, but unfortunately not in my size. There's always a chance he staged all that, but they looked unbelievable.