>>44675644chan has a high res forum. Landscapes are posted there from time to time.
Flickr is a good source as well.
Be careful with artistic photography, because they're often heavily modified in terms of their values and colors, especially when they apply an HDR effect.
And, a cheap DLR or cell phone camera can work pretty well for your own reference. I offen carry my DSLR around, and just take interesting shots, I have a massive folder of random backgrounds, trees, etc.
