My pick for guinea pig shampoo (or dog or cat) is HyLyt. It's a soap-free, moisturizing shampoo that is gentle enough to bathe an animal in daily if needed. When diluted a bit with some warm water, a small bottle can last quite a while. Great for routine bathing, or bathing for allergies/dry skin (let soak 5-10 minutes for this).
Hmm....you can order a 12oz bottle of it for $9.42 here: http://www.healthypets.com/hylsham12oz.html
Or an 8oz bottle for $7.95 here: http://www.healthypets.com/hylsham8oz.html
Otherwise, it's available through your vet.
A few months ago, I picked up three pigs from the SPCA, one sow and her two adult sons. They were in pretty good shape, but both of the boars had really nasty grease glands. When I shaved off the gunky, yucky hair, I realised they had open wounds under there. Of course, if was on a Friday night, so no vet, and the only thing I could think of using was Bactroban ointment. At first, I only wanted to prevent infection, but I was pleasently surprised. It worked wonders! Because it's oily, the ointment made the remaining gunk softer and easy to remove, and the sores healed very well within a few days.