Where in Boulder are you finding needles? I'm yet to see one after ~4 years here, but I've mostly stuck around CU campus and Pearl and the surrounding areas. Plenty of discarded vapes everywhere, though.
Boulder Creek is the most glaring spot, but you might have trouble finding them until the homeless campers tents are forcibly removed once every 9 months. Scott Carpenter is another good spot. Park East Park is where the aforementioned kid was pricked.