I think I've seen sports related buttons at Joanns or Walmart. Maybe those would work. But if its for a folk dance thing, maybe something plain would be better. I know you want to try to match your FOE, but maybe something that coordinates with it or compliments it would work too. Depending on time, maybe you could get plain white buttons and mod podge some little blue cars onto it. (either find a picture of a cute car online and print a bunch the size you need, or if you can find some stickers or patterned paper, that might work too)
Just a couuple of thoughts. Hope it helps.
I forgot to add that your FOE button bracelet is adorable!!! And just thought of another idea: What if you got a plaid button or made one? That would be more boy-ish.