Coupons.com has an offer for a $2/1 Robitussin coupon under Zip 77477! It takes you to Walmart's site, where you have to click "clip coupon" on the right side. Then, you get to the coupon print page. You should be able to do this twice and get 2 prints!
Walmart carries a trial sized Robitussin for $0.99, so this would be a moneymaker with this coupon!
Most other Zip codes on coupons.com have a $1/1 coupon, here's a direct link for that one.