Obvious free information sources include websites such as this blog and the forums at remodelcrazy.com and contractortalk.com. The advantage of the peer-based forums is you can quickly validate (or eliminate) anyone recommending a for-fee marketing solution. “Free” offers which are thinly disguised sales pitches for one marketing “solution” or another are hardly frugal answers!
The public library is also helpful. You won’t always find the latest and greatest, but many marketing principles are timeless and some of the classic books on marketing will certainly be available on the library shelves. (And if you want to “splurge” on the used book market, you will probably find you can obtain these books for just a few dollars. I especially recommend you read Positioning: The Battle for Your Mind by Al Ries (the link here is to Amazon.com and so isn’t exactly free! Nor is my own Construction Marketing Ideas: Practical Strategies and Resources to Attract and Retain Clients for Your Architectural, Engineering or Construction Business, but without an ounce of modesty, I believe it is a bargain, as well.)
Finally, consider your current clients as some of your most powerful free marketing resources. By knowing what your current customers read, enjoy and relate to, you can plan your marketing strategies to reach out for similar clients. Best of all, you may pick up some repeat and direct referral business as you complete your market research.