i've used them on my friends. And yes they do work, but sometimes not too well.
When I cuffed my friend's thumbs behind her back with them, even with the key she couldn't get herself unlocked, not having her thumbs made it really hard to do anything.
On toes, they suffer the same problem as normal handcuffs for bondage. They're static restraits. Even though handcuffs are connected by a chain or a hinge, the restraint itself is very thin and hard (metal). The thumbcuffs when used on toes are even worse, if you're going to use them on toes, you better make sure her ankles are secured together also because any excessive movement will hurt big time.
It all comes down to them being good for show but not good for actual play. If you're going to restrain someone's toes during play, you're much better off using some leather cuffs, kinda like the ones in The Last Laughs productions on the top left there. No bruises and no bad pain = a good day of play
To find them, i'd just go do a search and order them online, nickle-plated ones are the ones you're going to want to look for
The most recent times i've put them to use was with my friend in a little self promotion: