I don't know any stores at all since I never bought any...hehe
But, I know a website called www.airplaneshop.com that sells them, but they are a little too expensive compared to all the other competition. Then there is a great site of a world famous store in London, UK called www.collectorsaircraft.co.uk
And finally, Ebay has a lot them! Just type in the search "desktop model".
Also depends, which airline, but most sell 1/200 etc. scale models in their duty free on their flights...
Hope this helps,