You can find them at most large photo stores, like B&H and Adorama.
I have this one and it works well. I have NEVER seen these for 11x14. If you are short on cash, you could buy what is called a "quick easel" for 11x14.
http://www.adorama.com/PE1114AE.html These are also option for economy minded darkroom owners. You can get them cheap used or buy one new. I had one SIMILAR to this one. I tended to have problem placing paper square to the corner, so I eventually gave mine away. I bought 4 blade variety. They are much more expensive but to me, it's worth the cost.