@BJ Grove If you are working with an agent (or have access to one that is willing to assist you), I recommend asking them to pull MLS rental comps. Since MLSes are generally where all of the info on 3rd party sites are aggregated from, I typically find this to be the most accurate/preferred method for pulling rental comps.
If you don't have access to an agent, I think the best way to go about it is by checking what other landlords are charging around you as this will enable you to get a sense of your competition in real-time. Use sites like Hotpads, Apartments.com, Trulia, Craigslist, etc. to scope out your competition--this is what other tenants will be seeing when deciding whether to rent your unit or another, so it gives you the best idea of what you're able to charge based on condition, amenities, location, etc.
Running comps is an art as much as it is a science. If you can't find an exact match, always better to be more conservative when underwriting.