Good one, Kate.
I use cork every time now. Its pretty, light, durable and so easy to cut, or sometimes I just break it along the natural grooves for a nice rustic look.
Tropiflora usually has lots. I buy a big slab about once a year and just use hunks of it until it's gone. See if they have a pallet of it and they might let you pick out your own piece. Last year I got a lovely one for $24 and got 11 mounts from it. They sell it by weight.