It's a somewhat vague sentence; were you trying to explain that shingles, firewood, and wood products were previously produced mainly by farmers in search of extra money? If so, I would write: "In earlier times, farmers in search of extra money were the main source of shingles, firewood, and wood products." You could also say: "Prior to this" in place of "In earlier times" depending on what the sentence prior to this was. Were you also trying to explain that the supply of these wood products decreased significantly due to the change in the farmer's behavior/incentives?

I'm confused about what "the main source" does in this sentence. Do you mean that farmers need the extra income in order to afford shingles, firewood, etc.? If so, you can drop "the main source" since it's implied anyway. Also, I agree with Jonathan that "no longer were farmers" sounds a little formal and old-fashioned. However, I still think you could use it, particularly if you want to emphasize the change over time, rather than the continuity of whose needs you're describing. So, you could either say, "No longer were farmers in search of extra income for shingles, firewood, and other wood products," or "Farmers were no longer in search of extra income for shingles..." I hope that helps!

If you want to make it more conversational, you could start it, "Farmers were no longer..." The writing style is formal. "No longer" has been placed at the start of the sentence to give it emphasis.