With orders of $25 or more, Amazon offers Prime members the Plumb Pak Double Thrust Plunger, model no. PP845-7, for $3.73 with free shipping. (Non-members can spend $35 or more to qualify for free shipping.) That's $5 off and the lowest total price we could find. It features a 6" cup and 19" handle.
Note that this item is sold as part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.