Yes, Betadine is readily available. The label may not billboard that it is Betadine. It may say First Aid Rinse , etc. Just read the ingredients.
I have no experience treating scale rot. I can only give you an opinion on what I have read and this is how I would do it. Unless someone I respect here has valuable, different input from experience
I've read to make anywhere from a 1-5% solution bath in a tub. I wouldn't get hung up on numbers. Just stir a solution until it becomes a light funky brownish-orange.
Some wipe on with a towel and leave it.
I like the idea of the bath because as the snake moves in the bath, the solution can get underneath the scales as well.
Do it daily for 15-20 min, then rinse.
I don't like the idea of testing the water temp by "feel", as in lukewarm. Our body temp is 98 degrees. What feels lukewarm to us can be too hot for the snake. That's why some snakes will spaz out trying to get out of the water.
I use my temp gun an try to achieve 77--84 degree range if possible ( it will actually feel cool ). I find it less stressful the few times I needed to bathe a nasty snake. Like the time Lizzy regurged in her bowl and laid in it completely.
If no improvement in 4-5 days, consider a topical antibiotic. If that doesn't help, go see a vet for more aggressive treatment.
add:
The "bath is in a plastic tub with the solution deep enough to completely cover the snake. Then place the lid on.