Here are some links I have found helpful (but not complete nor completely accurate) in helping me find information on some flags and coat colors. This list is by no mean exhaustive or conclusive final words on colors of units.
Baccus Miniatures: They have some painting info on coats and flags.
English Civil War Society: Make take some “diving” to find info here.
The Sealed Knot: Great Britain’s big ECW re-enactment group. Great info here!
Flags of the World: Good info especially on trained bandes
Flags of War: Sells miniature available flags.
Battle Flag: Company selling miniature flag sets. Good Irish Confederate flags here.
Archive of the Colors: Has some flag definitions and some flags you can probably easily clip art for your armies.
Wargames Designs has lovely flags for sale.