Least expensive is not to buy at a shop tbh. I joined several local kit swapping FB groups and that is fulfilling all of my plastic needs. Might take a while before what you need comes around, but WW1 aircraft here go for 2-3 euros for a Revell one and up for the more rare brands. Or open a wanted topic and see what the masses throw at you.
Other then that, I enjoy the services from Modelhobbies when I need someting.