Hi joesh,
Locally you may want to try Flexus, Selectabed, PJ’s Sleep Company, The Clean Bedroom, or Good Night Mattress. Some of the others may also carry toppers but they don’t list them on their site and I don’t know if they have any off the top of my head.
If you are looking online then the component post here has some of the better online sources for all types of toppers I’m aware of.
Phoenix