Actually, Realtors can help you find rentals. I own a rental property, and for the past 10 years have used a Realtor I know (who specializes in rentals) to find tenants. Honestly, though, a lot of real estate agents don't focus on rentals because the commission is much less than making a sale. Commissions are always negotiable, but I happen to pay one month's rent as a commission to find a tenant--you may be able to negotiate a better deal (if you're a landlord; as a tenant, you often wouldn't pay anything). But some agents have made that a niche that they focus on, and they can be excellent at that. So I wouldn't rule out using a Realtor to find rentals.
Lots of tenants, and plenty of landlords, also use Craigslist. I've heard some success stories there. However, my personal experience has been terrible. And there are lots and lots of scams aimed at tenants on Craigslist. So--consider using Craigslist, but be very careful.