Rentals, just like properties for sale, can appear on the MLS or can be done without a Realtor.
Therefore, you need (or at least would highly benefit) from having a Realtor assisting you in finding properties that are on the MLS. A Realtor can also check rental comps--just like sales comps--to determine whether a proposed rent is reasonable.
However, more rentals are handled privately that are properties for sale. So while you might buy a property and restrict your search to the MLS, you may find it useful to make sure that you're also considering properties that aren't on the MLS. There are some sites--they tend to be local or regional--for rentals. Another avenue is Craigslist and services like those.
So, there really isn't one "best source" for rental information.