Join local REI groups. Contractors will often be a part of those groups, or other members may have contractors they recommend. But finding a good contractor is difficult. There are lots of bad contractors out there; fewer good ones. And many investors may not want to give you the name of their best contractor - they don't want him to be too busy next time they need him - so they'll give you their #2 or #3 guy. Some things to think about:
1. Good contractors are busy. They probably don't have time to run around with you to a bunch of houses you're thinking about making an offer on. You need to learn to estimate reno costs on your own. There are multiple books and resources here on BP.
2. A good contractor should be able to provide multiple references, or let you walk their current job site to see their work. Everybody says they're great. They may even show you photos that look good. But you don't know if those photos are really their work, or if the shower looks great in pictures, but the shower pan leaks. As Reagan said, "trust, but verify".
3. The ideal contractor is also an investor himself and understands the price pressure of REI. He'll know where to save money and where it's not safe to cut corners. Admittedly, there aren't that many of those guys out there, and the ones who are stay busy.
4. Understand that good contractors view newbie investors as a pain in the ***. There are always huge promises of how much business the investor is going to do, and requests to cut corners where corners shouldn't be cut, and when the contractor can't cram $50K worth of reno into a $30K budget, it's somehow the contractor's fault. Don't be that guy. If you're fortunate enough to find a good contractor, value his time and make yourself a pleasure to work with. You literally need him more than he needs you. Even terrible contractors have more work than they can handle right now.
5. Going back to "trust, but verify"....you probably don't want to get to where you only ever have one contractor bid your jobs. Then it's easy for the contractor to take you for granted and let his margins creep up and up. Ideally, you'll have two good contractors bid each job, and you'll let them know up front that you're always going to do that. Keep it professional and they'll respect you. But don't do the BS of having a dog and pony show where you have a house open for an hour and invite 15 contractors to come bid. They're all going to look at that and decide it's not worth their time.
Good luck with your journey.