B&H Photo is a good great source of International Unlocked Lumias.
Bing for "buy <model name>" and see what comes up. Check the websites for reputation before pulling the trigger. Many national chain stores have unlocked phones n their websites - even places like Target and Walmart.
For used phones I like the MarketPlace ads right here in the various Mobile Nations Forums. (Windows Central, Android Central, iMore, Crackberry, webOS Nation, Connectedly) You would have to be a member to get in on that though as it requires sending PM messages and posting. Also Swappa is a pretty trusted site for used devices of all kinds.