It looks like Andrew answered a few of the questions you've asked here, so I'll just fill you in on the rest.
You can pick some holy water up at a local church. They may have some stocked up at the complex too; I would check around with some of the people who live there. If worse comes to worse, though, come to me and I'll bless some for you myself. I'm not so sure that they keep it stocked in many places outside of the church, so don't expect to go walking into a grocery store and finding some. It's not something that people use on a daily basis outside of religious reasons, as demons aren't creatures that everyone is aware of or interested in believing in.
Me and my brother, Dean, will help you with the hunting. It's a trade you definitely need a lot of practice in before hitting the field, then even more so before you can tackle hunts alone, but we'll help you along when you feel that you're ready to begin.