The Burdizzo castrator does not appear to be a stock item at these stores. Don't be mislead by a similar device used for fastening ID tags to cow's ears!
I did find Burdizzo devices for sale on the Web. Just Google "Burdizzo."
However at TSC and RK, I did find many books and pamphlets on animal husbandry that include castration information. From what I have read, the Burdizzo is somewhat out of favor versus the use of a bander, but both are mentioned as ways to castrate farm animals such as sheep and cattle.
As for the use of a Burdizzo on a human, I will recommend against it's use due to the damage to tissue and the resulting danger of infection. However, if you are determined to use such a device, then the size suitable for sheep would be a good choice. Basic instructions given in the various books that I read, all recommend a Tetanus booster shot be given. Since a barnyard is not a sterile environment, that sounds like a wise precaution.
Each testicle is to be done separately. They do not recommend trying to do both at the same time as it will increase the risk of infection due to extreme scrotal damage. Pull the testicle away from the body and make sure that you can locate the cords. Then apply the Burdizzo and clamp the entire cord firmly in the jaws. After 45 to 60 seconds, release the instrument and repeat on the the other testicle. The testicles will wither and fall off after about 2 or 3 weeks.
All this sounds very simple, but here's the rub.... The books say that the animal will be stunned for a few minutes, but eventually will get up and run off as if nothing ever happened! I doubt that this would happen with a human. Be prepared for a LOT of pain. Also, your damaged testicles and scrotum will not be a pleasant sight. You should be prepared for a very embarrassing and expensive trip to the Emergency Room at some point to have things trimmed and sewn up properly.
Don't try this at home.