First check to see if your county or state historical libraries have them online. Some are available to library card holders for that particular area. Used to be that some folks who had access online would share pass codes but you don't find that any more.
Not hard at all to do an online search. I found several online sources for my area. Be aware, however, that Sanborn Fire Insurance Maps were not made for every community.
My local library has Sanborn maps of the area on film. I just head there and scroll through till I find what I'm looking for. Then I can send digital copies straight to my email and have them on my phone and iPad. It's nice because with the digital projectors you can zoom in and out and get digital of exactly what you want.