Go tot the local veterinarian and have them write you a script to take to the local pharmacy. Explain what you need it for and tell them how many gallons you are treating and have them write out the script for 50-60mg per gallon instead of the 40mg/gallon. It won't be the cheapest med you have ever bought but it will be 99.9% pure. Also see if they have a card behind the counter that will discount the medication, since if you don't have pet insurance your insurance won't cover it.