If you could provide photos of the attachments, and the year of the boat, someone may have some thwarts gathering dust in their garage that will fit your boat. Otherwise, you may need to buy some new 35" long thwarts and retrofit your Avon with them. I'd recommend that if you have to retrofit, you look for some Hypalon thwarts so you won't have to deal with patching gear made from PVC as well as your Avon.
Also, I presume you're only looking for two thwarts, and not two sets of thwarts, right?
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse