I buy various sticks from Small Pet Select.
I also have a large supply of willow baskets & tents from Binky Bunny. They have new (cheaper) palm leaf bowls, too -- I picked up around 50 of them recently when I found my rabbits like to eat them.
I had a cardboard box that an xpen came in that my buck loved to eat. He probably ate more than half the cardboard over a couple years. As long as the material is mostly natural -- no glues, coloring, etc., it shouldn't be an issue. I've caught my doe eating the cottontail cottage, too.
During Christmas, I always buy small trees exclusively for the rabbits to eat. I get mine from the grocery store. For $25 (half off after Christmas!), the 2-3' tree is a pretty good deal as far as edible rabbit chew toys go.