The cheapest I see is through my credit union. If you belong to one (or know someone who does) see if they have member discounts that include DLR tickets.
Otherwise, lastminutetravel.com, ares travel or check Orbitz using an Ebates rebate.
I would get them at Getaway Today. If you buy your hoppers now, you will still get the old price. Their prices are going up on the 8th. Also, if you buy a 4 day hopper you will get the 5th day free.