I use Lugol's solution, which is mostly available in pharmacy, and some pet store also carry them.
Normally to use as supplement (not recommended though, highly debatable), we dilute lugols to fresh water by 1:4 first before administering to tank.
If you want to use as dip, I would use about 20-30 drops of lugol to a gal of tank water. Because the iodine concentration is very high, you should really just "dip" it for a few seconds.