You can try eBay Http://www.ebay.com
(for you I would suggest that you buy a lot from one person so the shippling charges are not too much) The Airplane Shop http://www.airplaneshop.com
but I would suggest Schabak USA http://www.schabakusa.com
because their prices are cheaper
than The Airplane Shop, and the orders are transfered to The Airplane Shop, so there is no need to buy from The Airplane Shop if you want Herpa's.
Otherwise, if you ever know one anyone who is going to Honolulu there is a shop in one of the terminals, although I am not sure if they have Schabak and their prices are a little high. (I know that they have Herpa's and the kiddy airplanes, although their stock might change, and they might be there at all anymore - who knows!) Also, Hamley's in London has some models.
Hope this helps