try this link: they are homeopathic and clear, tasteless, odorless, and you just add 4-6 drops in their water dish (approx 4-6 drops per 2 cups of water) http://www.pets4homeopathy.com/remedies.html
look on their for things like the INFECTION treatment and DETOX (that one you should use in conjunction with any of the mixes) and search through until you find one that will help tonsilitis, you might have to search around because they might have one not specifically called TONSILITIS, I know there's one for respiratory, but it may not be listed on that page, keep searching that site, it's got EVERYTHING.