Avon tyres are completely unobtainable new, regardless of the whispers put about by some folk.
Most five-inch tyres in the UK are all wheelbarrow-type tread, slicks or mobility scooter tyres, so for (more or less) authentic road-going knobblies, you have to cast your net to America.
Head over to eBay.com and search for "3.50 x 5 tire", and behold the choices...
I used Kendra 4.10/3.50 x 5's, at the equivalent (four years ago) of £7.50 each.