You can get betadine liquid from horse supply stores. I don’t know what you would have to do to it to make it a nasal spray.
I thought that it was for topical use.
Betadine is used in hospitals, the brown stuff the pour on patients in surgery and swab them with. You take that stuff, two teaspoons in 300ml of water, spray it in your nostrils with an atomizer, then snog it to the back, spit it out, gargle the rest and spit it out.
It is medically approved for use in diluted form in the nasopharynx and has had papers written up on it and deemed safe, but I wouldn’t be surprised to see it outlawed.