I have fitted an inflatable floor - it fits both my 2 old (1979 & 1984) Redcrests. It is a very good fit although needs standing on initially (afloat) to get rid of a crease when inflating - I suspect it's just a fraction too big in the beam to be a 'perfect' fit but it doesn't seem to be straining any seams. I'm pretty sure that it is a Zodiac floor, but I bought it from a stand at Beaulieu Boat Jumble about 4 years ago. It was marked (by the vendor, who had various sizes) as being 6' 6". I paid £45.
I think the floor was originally for a 'sportboat' shape dinghy as there is access to a keel inflation point and twin 'flaps' at the back. I'd recommend it - it transforms getting in and out and keeps any errant water below the floor and away from the feet. If you find a source, let me know and I'll get another!
Avon is part of Zodiac so if you want to get ripped off I'm sure from my own experience that Zodiac will oblige with a trade in offer which sounds good but might just be the dealer being ripped off from his margin IMHO.