Why not use a deep indigo midweight denim? That's what they always seemed like to me.
Failing that, I suppose a blue twill would also work.
is a good site- their prices are very reasonable and they offer swatches. Denver fabrics
online site are the two other sites I use most for ordering fabric online, though beware that neither offer swatches (Denver Fabrics TECHNICALLY does, but only for a very few fabrics..).
Here's a good denim from fabric.com that could work.
And a twill.