They're easy to make with a food dehydrator and tasty, too (I have this on the best authority, and I don't mean Charlie ;) ). 10 pounds of yams makes 2-3 pounds of chews, so they're even economical, compared to buying Sam's Yams. I blogged the recipe before. But since then, I've made an improvement -- perforating the slightly cooked yam slices before dehydrating them; it really makes the drying go better and faster.
Here's the short version of the "big dog" recipe:
1. Buy a bunch of the biggest yams you can find. 10 lb. fills my 6 round dehydrator racks.
2. Slice the yams into 1/2" - 3/4" slabs.
3. Cook the slices on a plate in a microwave -- about 4 min. on each side.
4. Perforate the slices all over (on both sides) with a fork
5. Dehydrate @135-140' F for 10-20 hrs.
6. Feed and repeat.