If you are looking for newish stuff, then copyright law prevents complete free access. However, given the late 18th, early 19th Century is "public domain," the originals from that period are freely available.
For instance, at the library I found a letter from William Livingston, Governor of New Jersey (1776–1790) during the American Revolutionary War and a signer of the United States Constitution, where he seems to deny the Trinity to the very orthodox Jediah Morse.
And when I came home, I found it online via googlebooks:
"Rev. Sir: I received your letter of the 26th of October yesterday. Since I sent a description of three of our Counties to Mr. Whittlesey, (whose death I sincerely deplore,) I have received that of one or two others, which shall be at your service, when you do me the pleasure of what you have given me the agreeable expectation,—I mean a personal visit at my Hermitage, alias Liberty Hall, in the vicinity of Elizabethtown.
"That I have received the descriptions of so few of out Counties as you mention, I now find, or at least am told, is my own fault. Although I had a number of copies made of your queries, immediately after you delivered them to me last fall and, as I thought a sufficient number to give one to each of out Council, yet some members of that Body tell me they went home without one, and that I promised to send them after the rising of the Legislature; but that they never received them. If the case be really so, (of which, however, I have not the least recollection, nor greater faith than I have in St. Athanasius!) I can atone for my neglect only by delivering them at our present sitting, and pressing those members to transmit to me their answers as speedily as possible. The Legislature expecting to adjourn next week, it is probable that I may receive them seasonably enough before your intended publication. ..."
In short, Livingston was accused of making a gaffe. In the context of saying he didn't have "the least recollection" of the incident, he said had no greater faith the incident happened than he has in St. Athanasius. St. Athanasius, of course, was to the unitarians of the Founding era, the man chiefly responsible for fabricating the Trinity.
The notion that I'd sooner believe in the Trinity than I would that, seemed to be a running joke among men involved in the Revolution. We've already seen evidence that Baron Von Steuben made the same joke to Timothy Pickering (causing Pickering to become a unitarian).