Although you can buy the Ivomec paste in some feed stores, you have to be very careful with it. i.e. you need to empty the entire tube and mix extremely well, before treating the ratties. They only get a dose that's the size of a grain of rice.
I've never used this on my rats though, as it really makes me nervous.
I have found it safer to go directly thru my vet. They have Ivermectin in a fluid form that you can put on a piece of bread, or in a dab of cool whip. They can also help you with the appropriate dose, based on your rat's weight. The downside of Ivermectin for me, is that you have to give it in at least 4 doses (most vets say 6) at one dose once a week for 4 weeks, to be affective. Even then, I haven't had the greatest results.
We use Revolution on our rats. Of course, it has to be carefully measured based on the rats weight too. But for us, we have a scale so that's easy. A tube of Revolution can treat several rats, depending on their weights. But if you are comfortable with weighing the ratties and calculating their doses appropriately, it works out really great. With the Revo, you just dose them once, using a syringe (without the needle) on the back of the neck (just like with cats and dogs). Depending on how bad the lice/mites are, they might need a second dose a month later, but my guys have always been good to go (we've only had to treat preventatively, when we brought in a rescue that was covered in lice).
If you decide to go with Revolution, the cat or kitten formula works best. And I'd be happy to help you calculate doses too.