You can use Zillow sold listings for rough comps. Find similar square footage homes, similar land size, and same bed / bath count no more than a mile from your subject property. I would also suggest joining a local REIA group and meeting investor-friendly realtors. Take them out to lunch, share your vision, and find a way to benefit them.
From a realtor's perspective, doing a lot of CMAs (time), and then showing a lot of houses (gas money and more time), and writing a lot of lowball offers (more time) that most likely won't be accepted (we all know it's a numbers game) means a lot of effort with the final result - if they're lucky - of scoring a very low purchase price property and getting a tiny commission that they divide with their brokerage.
In contrast, if they work with serious retail buyers, they are often writing more realistic offers on larger-ticket, pretty homes. Most realtors work with retail buyers over investors because they get paid more for much less work.
However, if you can show the realtor they will do more sales because you're an investor (volume purchases vs one-time retail purchase), you may be able to change their mindset. For flips, if a realtor were to bring us a deal (low purchase price means low commission), we would make sure to use them as the listing agent after we've fixed it to a retail price when we sell it (better commission is reward). That way we win on the low acquisition price and they win on the retail price as a listing agent... as well as sell the same home twice.
My advise (as both an investor and wife of a realtor), would be to use Zillow sold comps as your first analysis tool. Also, ask an investor-friendly realtor to set you up with an automated MLS email based on your specific buying criteria. (This should take them very little time.) From this list research homes that may work, then drive the neighborhood and drive by the house to make sure you're still interested after seeing the outside. If the numbers and property drive by work initially, then ask a realtor to run comps and, if those still work, to show you the property. Or, if you are doing a lot of low offers with very little chance of acceptance, instead of making the agent show you every home first and burning them out, just have them submit offers for you contingent on inspection / financing / partner approval, etc and estimate a worst-case rehab into your offer price based on the pictures, MLS notes and disclosures, age of the house and square footage. Once an offer beats the odds and is under contract, if it's somehow even worse than you estimated and the numbers don't work, you can always pull out because you've worked that escape clause into your contract.
I would also get a pre-qualification letter from your lender. Realtors want to know you can close; and having financing lined up shows you're serious.
Be respectful of the realtor's time, show them how working with you brings them more business (volume / referrals / retail sales) and it will go a long way to keeping them engaged.
Good luck!
