Houses are typically rented out by the owner. In some areas you can find property mangers that specialize in houses. The owners that manage the rental themselves often only advertise in free places like craigslist or the local paper. You can look in the yellow pages for a property manager in your area that does this.

and NO $800 is NOT unreasonable in some areas it all depends on location!

Depends on location, but $800 is pretty low for a house in most areas.

If looking for an extended period, try a local realtor. If short term, there are a few websites out there, just search for your question in a search engine.

www.realtor.com and search by city

exactly what do you expect to find for $800/mo? there are plenty of houses for rent in your city/locale - the problem isn't you not knowing wehre to look for them but instead having unreasonable expectations in your demands (ie $800/mo).

Online, in the paper, friends