Owners who've moved and their homes are vacant: Just search the MLS for listings with the comment or notation "vacant." And don't overlook vacant rentals, either.
Owners on vacation: Network with travel agencies. Let them know the service you're offering (peace of mind for someone on a long trip). Possibly offer a finder's fee. Or you might not even have to do that if the peace of mind is enough to persuade some people that it's safe to go on a trip and leave their homes.
As far as how to price those services: It depends, in part, on your objective. Is it to make money? If so, I'm not sure there's a huge opportunity. House sitters usually get paid fairly little. What they're getting in return is a nice place to live for a while. If you're trying to make contacts (with owners whose homes aren't listed), then it might make financial sense to charge very little. Make a good impression, establish a good relationship, and some of those folks will come back to you when they're ready to sell.
Hope that helps.