Goodman's and The Gourmet Depot both sell replacement gaskets for blenders. Prices range from less than $1 to as high as $10 as of 2015, depending on the brand of the blender and the store selling it. Without a gasket, blender contents leak quickly onto the blender and counter.
Blender gasket size is fairly uniform for many brands, but it's wise to check for the model number and manufacturer before placing an order. Goodman's carries a replacement gasket that fits dozens of different Cuisinart models, for example, but is not listed for other blender brands. People who have multiple blenders or use their blenders heavily might consider buying a two-pack such as the one that The Gourmet Depot offers.