Hi Conne:
Don't know if you have a branch of Wilkinson's near you but that's where we get some extremely robust rubber balls for our dogs. They cost about 70p each.
Our two actually chase them as well as opposed to simply chewing them, although they do that as well.
The balls are obviously small enough to get in their mouths but not too small as to be a danger of choking them. And they usually last at least a month, using them on a daily basis for about 40 minutes at a time.
Hope this helps.