Any providers have them in their inventory for play....seems very nich for most.

Never met a domme who used either.

of anyone what had either of those instruments and me naked and restrained and holding my testicles in her hand.

does not know how to use them you can be in trouble.  You can buy them at most farm and garden feed stores.  Price varies depending on quality. The elastrator  stretches a small rubber band which is then released around the testical  cords to cut off circulation.  The burdizzo crushes the testical  cords in one motion.  Years ago I read about a provider in Chicago who used an elastrator and ended up leaving the band on for too long.  Not good. A 4H girl would be very familiar with using these.

I have an elastrator and have used it, however I have been doing CBT for over 20 years. The elastrator can stay on for no more than 8 minutes. If left on longer, tissue necrosis begins, causing irreversible damage.  
I do not advise Mistresses to attempt using it without proper training.