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You can find the Pharmaseb or a product similar to it at your regular vet's office. Because it's a medicated shampoo, odds are that you're going to have to get it from your vet.
Something you might want to consider would be to go through 1800petmeds.com. They're going to require a prescription, but they may be cheaper than getting it from your vet. http://www.1800petmeds.com/ .
You could also try getting it from the company that produces it. They may or may not require a prescription, you'll have to ask them, but they do carry the gallon size:
You can also try places like Dr. Fosters and Smith (http://www.drsfosterandsmith.com/ ) to see if you can find a product like that one.
I hope this helps.