I ran into the same problem last year, but eventually things get restocked and you can get going again--although it does delay things and is frustrating. I simply bought what I could, when I could, until I had everything I needed. I also bought at least three tins of each color, to carry though the lean times.
It looks like everyone is out of 20-B Deck Blue (including WEM themselves), but here's a link where at least 5-N and 5-H are still available in the USA.
http://shop.warshiphobbies.com/WWII-US-Navy-Colors_c360.htm
Fair winds and following seas.