The Do Not Pet collar can be bought from Stillwater Kennel Supply, in their Service Dog Collar section of their site!
They also have “IN TRAINING”, “SERVICE DOG”, and “I BITE” in sized 1”, 1.5”, and 2”
SWKS is based in Alabama, ships worldwide, and can make custom-sized collars for you (Floyd’s neck is larger than their standard collar size so I had to custom order)
Edit: For some reason the In Training, Do Not Pet, and I Bite collars aren’t appearing on his website for me. He does have them though, you’ll just have to email him.