I agree with this comment, you can get those silicon rubber (reddish) O-ring from Preston Hardware at Preston street and costs less than $2 for 10 pieces. You do not need to bring the parts to anywhere, ask the service person in the store for garden hose O-ring. And then what you need to do: 1. turn off the main valve of your water supply, 2. open this valve in your backyard, 3. anticlockwise turn the hexagonal nut with a runch and take out the old O-ring with your finger or a screw driver, 4. put a new silicon rubber O-ring and clockwise turn on the nut, 5. close this valve, 6. open the main valve of your water supply. You will need not more than 6 minutes to complete all these steps.
good luck!