The only other option I have found looks like a decent one. The Dunlop K81 4.10 x19 fits our standard rims and is a universal, like the Avon. Unlike the Avon it has generous tread depth at 9/32" , which is about 7.1mm, similar to dedicated rear Avon 18".
When the current set of Roadriders wears out I'm going to give the k81s a try. Fortnine has them at a bit lower cost than the 100/90/19 Roadriders.
I like the Avons on the Norton in general but don't enjoy the need to change them and pay for them on quite such a regular basis!
Glen