To my knowledge, RedCap doesn't sell direct. You'll need to either check with a local uniform store or do an online search for uniforms and coveralls.
One of the drawbacks is that if you want something unique like a solid black or white coverall, you might have a minimum order of 6, 10, 12, etc of coveralls. I wanted a black coverall for my Biker Scout costume and wound up getting a solid white coverall form Gempler's, the same place that sells the correct rubber boots on clerenace. I'm dying the coverall black.
Ritz Dyes work but you have to follow the directions to the letter...use VERY hot water and salt.
Here's a link to an online uniform store that sells the RedCap brand coverall in postman blue:
http://www.theuniformstore.com/store/product.asp?dept%5Fid=10&pf%5Fid=5&mscssid=1CUEBQV45NSR2G4700JP48DLPKWNF8WD
A 100% cotton version of the coverall is also available in the same color...avoid it...it'll fade and shrink quickly...go with polly blends...ditto for the work shirt if you go the route I did.
Good luck, 
Neil