I can't believe I've gone this long without a ham holder.
These things aren't easy to find, either. Well, I did find a wooden ham holder at my local fabric shop - with a $25 price tag. Clotilde also sells a wood one for $25. I found one online source selling the June Tailor plastic ham holder - SHC Fabrics has it for $17.
I was determined to get a better deal. Eventually I found my way to Mary Jo's Cloth Store where they sell a wooden ham holder for 12 bucks! I'd never heard of this online store, and navigating around the site is annoying, but I can report that my order was processed quickly, so I am pleased and I'll gladly recommend the site to all of you still 2-handed seamstresses out there.