So yesterday i was working (i work at a fast food res)(not saying where

) then a customer ordered a meal. So the total was 15 something. She hands over a 20 Dollar bill. There is this marker that we must use *mandotory* on 20, 50, 100 dollar bills. So the 20 was fake it turn black. *which means fake*
The lady was like WTF .. all scared, but i have a question. Where do this fake dollar bills come from? Is there more out there?