Have you seen the caps Cooperstown Ballcap Co. used to make? They actually recreated the real style of the caps era appropriate, short bill, lower crown, etc as well as the french-seam box cap. The caps were painstakingly recreated using originals as a guide and all hand made. The company is gone now, but Will the owner still makes caps under the new company called Ideal Cap Co. He's branched out to other types of caps, but he can still recreate any cap you want and his data base of historical styles is unreal - I know because I use his old catalogues when I do my own baseball drawings. Ebbets makes a damn good cap, but they make it in the post 1950's style, which if you want originality just isn't right. If you want his email to talk to him, pm me and I will pass it on to you.
Here is a blog someone started who really likes those caps:
http://coopcapco.blogspot.com/
If you go the February selection of posts it is all Negro League caps so there is a lot to choose from.
For actual pictures of the Negro Leagues, the website:
www.baseball-fever.com
has a section with all negro league photographs in the "Baseball Photography" section.
Just google "cooperstown ballcap" and go to "images" and you can see other caps they made. By the way, the more worn in they get, the better they look!