No, you certainly won't find anything like that online, at least not for free.
If you don't need your poster to be super fine then you could just print at 72 dpi. A 3' by 2' poster for example would be about 12-15mb, which is around a 1700x2500 pixel image(ok, so thats a pretty big poster, but just an example). While it's still unlikely you will find that online for free, you might get lucky. You could shoot that with a 5 or 6 megapixel camera that you might happen to have.
If you need a really superfine image on the other hand, you are probably gonna be looking at pro shots or something. The same size poster as before, when printed at 300dpi (photo quality) can be around a 10000x7900 pixel image which is over 200mb. Thats a fat file! Not likely that you will need something like that for a local fair tho.
There are a few websites round that offer free textures/images without royalty, but there are still restrictions on use and therefore you would need to read their EULA or whatever closely. Mayang's Free Texture Library is one example. They offer a few nature backdrops, but i'm still not sure they would be adequate for print nor would they really be what you are looking for.
I hope this helps some.
Edit: A 5 megapixel camera is probably not going to get you a decent print thats anywhere over maybe a foot & a half wide and tall. My camera is 5 megapixels; my brother printed a 3 or 4 foot wide (don't remember exactly) poster shot by a 10 megapixel camera. Just an afterthought.