Where do IT recruiting companies get leads for open positions?

I'm assuming that some of the job leads come from the big jobs sites (Monster.com, CareerBuilder, Dice, etc). However, do recruiting firms reach out to companies to get on some sort of "vendor list" so they get notified about jobs?

Note that I'm not interested in finding the individual contractors, but the actual contracts themselves.

We don't find the big job sites like Monster and Career Builder that useful for finding vacancies. For us it's about doing our research and locating growing local companies contacting them and selling our expertise. Most companies think you need to pay a recruit upfront but that isn't the case you only pay once you have chosen a candidate. As mentioned in the previous answer some companies have PSL's but these tend to be public regulated bodies or you major multinationals. The best advice we can give is start local and build your reputation.

Yes, recruiters make the contact with companies (cold calling, email marketing etc) to sell their recruitment services. If successful they will be added to a preferred supplier list for their service type (IT staff, sales staff, permanent, temporary etc). When there is a position to be filled the companies place the jobs with the recruiters. They will then find staff to fill roles and submit them to the company for consideration and then interviews.

Some roles are advertised directly by the company themselves but they rarely want to use subcontractors or other companies to fulfil that need.