I am having trouble finding Maple Bitters, even online. Where did you get yours?

I would suggest a quality liquor store. (Not some generic corner place.)

If you have specialty stores (wine & liquor, gourmet provisions) nearby, ask around. Same for bartenders at top hotels and restaurants - where do they get their fancy bitters?
Last ditch source - Chowhound has a recipe for homemade maple bitters. Haven't made it myself, but it both sounds good and only requires one hard-to-find ingredient.