As Diana mentioned, the hardware store would be your best bet. Try the water purification aisle (i.e. where the water softeners are normally kept) because potassium permanganate is usually used to recharge the iron removers.
Or try looking up a local chemical company. Potassium permanganate has many uses in fish, especially koi ponds. You might be able to find a local pond/watergarden store with something called Pond Permoxyn, made by Kordon. (its just a branded KMnO4 liquid)
Thanks for point out that site. Started clicking around and realized that he stole my photo to sell his nerite snails. Robert Paul Hudson even went as far to crop out my signature! That is a GREAT way to run a business.