Weird. I’ve been googling for a site exactly like this: A free source for symbols (or pictograms to be more precise) and could not come up with one. So here it is:
Map Symbols & Patterns for National Park Service Maps. As this is an US government site, I assume these symbols are free to use. I couldn’t find anything regarding copyright on their site, so drop me a line if you have details on the usage terms.
The site features different sets in .pdf and .ai (Adobe Illustrator) vector format. Enjoy!