Honestly, I use Google Books a whole lot. You can find books from respected academic historians on there as well as books written during the time period your researching. For an internship I completed last summer, I researched food in the Midwest during the 1930s. I used a few USDA published articles during the time period for my research and I would've had a harder time accessing them if they weren't available through Google Books.

The bad thing is that sometimes they only have portions of books available for preview or don't have any preview pages at all. But they do usually have a link on the side titled "get this book in print" and another link under that tab titled "find in a library." That link will take you to Worldcat, which tells you which libraries have that book and it's organized to show the libraries closest to your location. So if Google books doesn't have it and your very interested in it, you still might be able to find it in a library nearby.

For a free website though, I think Google Books is fantastic

Your local library's website. Aside from paper books, which still exist, they may provide access to databases like JSTOR, which offer academic journals and papers, as well as eBooks.

I always recommend H-net and its reviews to people. Written by historians, they review books on virtually every subject you could be interested in, which you can then get from you local library.