Search for a mattress topper online. Type "queen foam mattress topper 2 inch" into your favorite search engine.
Compare prices from various websites. Find a brand you like at a price that appeals to you and order it. Specify you want a queen topper, if that's your mattress size.
Purchase a mattress topper locally. Look in your phone book for furniture stores and mattress dealers. Call and ask if the businesses carry 2-inch queen-mattress toppers and how much they charge.
Pay for your mattress topper in person. Arrange for delivery or transport it yourself.