Most any larger department store like Sears, Target, WalMart, KMart, Herbergers, Kohls, Bed, Bath & Beyond, etc. carry mattress toppers.
We tried one on the factory camper excuse for a mattress and it didn't help. You can't polish a turd as they say. We found the topper very hot too...so dumped the factory mattress and topper and got a regular home mattress. Great choice for us.
2012 Fun Finder XT 276 Toy Hauler & Harley Davidson Ultra Classic with Condor wheel chock
2014 Dodge Ram 2500 AT 4X4 Crew Cab 6.4 L Hemi, 373 gears, tow package, Rambox option, Revolver X2 tonneau cover, Equal-I-zer anti-sway hitch.