Use Zillow. Their zestimate is based on some robot comps. You don't need to trust it implicitly but it will give you a decent range. Turn-on display of recent sales; note the details of similar units and date of sale. Ideally, enter some of these in a spreadsheet.
Look up property tax assessment. Again, the absolute value isn't critical but look a the trend. If necessary, look up the same tax assessment records for some of the Zillow comps.
Look on craigslist for FSBO comps. They will tend to be high.
This one is sketchy... if there are bandit signs around, call the wholesaler. Make up a story about a fictitious comp. If they demand a starting price from you, start real high. The wholesaler will under-shoot market value.
Become an expert on your area. If you study the MLS, zillow and comps you'll eventually become an expert. The smaller the area, the faster you'll learn. If you're area is large then it will take a long time.
Finally, consider working the deals from both ends. Simplified, purchase price & rehab & rent => cash flow & profit. Instead of using your numerical analysis to determine precise rate of profit, decide on an acceptable rate of profit and re-arrange the equation. You should be able to calculate a max purchase price. As long as you're meeting profit projections, who cares if you don't know the exact value.
