As /u/Nm0369 said, elk antlers are the way to go. I've bought two off of Amazon.
This one was $19 and my Berner loved it and still pulls it out every once in a while. It's a good size and is really really cheap for what most elk antlers go for.
This is another one I bought that's $28 and it was good too. I thought it would be a little bit bigger than the other one but I think it was actually a bit smaller overall. It's more cone shaped and pointed at one end whereas the other one was a chunk of the middle of an antler.
I'd say my Berner liked this first one a little bit more.
Hope that helps!