Ivermectin is used for both guinea pigs and cattle, at the same dose rate. The difference is that a guinea pig is a 2 pound animal and a cow is a 2000 pound animal, so the pig gets 1/1000 as much of the medicine as a cow does.
The paste is usually $5-10. You want a paste that has only ivermectin in it, no other active ingredients.
Wow, I just saw that at (broken link removed) they have some crazy cheap paste!
(broken link removed)
The Horse Health, Zimecterin, Equimectrin, or Agri-Mectin are all the right product. You will need some 1cc syringes ( (broken link removed) ) and a sealed bottle you can mix it up with water in. A tube of horse paste will give you 5 doses for all your pigs. It has a spinning thing that you can set at each notch or number so when you squirt out the paste, you only get 1/5 at a time, which is what the GL instructions are calibrated for.