Good soil for a garden in the Woods?

I'm going to be living in the woods on a lake this summer, I have some lettuce plants and tomato and cucumber plants and I will be making a bed by the water where their is plenty of sunlight. But I am on a budget and need the cheapest way to get good soil. The woods has some wet parts, and I'm wondering if I can use soil from the swampy areas, as it may be decomposed and full of nutrients. Or must I go buy soil. Anything is helpful, thank you!