I've made a few prints for my father with scans downloaded from the LoC website. They're dirty and scratched (as anything 150 years old and hastily scanned is wont to be) - but LR's spotting tools can clean up the worst and the scans are large enough to make nice prints.
I've also worked on some WWII/New Deal propaganda posters and Civil War-era maps for my own display, but Costco wouldn't print them at poster size for me (copyright concerns). Even printing out the LoC info about them being in the public domain didn't work. Wondering about an online print shop that might be an option, but I haven't really looked.