I recommend the reference book "The Source A Guidebook of American Genealogy" by Szucs and Luebking. This is a VERY comprehensive guide to locating sources in the US. Many of the documents for sale on Vita check and others can be found for free or at considerably lower prices if you are willing to send a letter.
Check the reviews on amazon.com. If you watch the used book sellers sometimes you can find it dirt cheap. I got mine for $4 + Shipping. Its my source bible and the place I check first.
Good Luck!