I recommend going to Amazon and searching for "mattress topper". A lot of different ones come up, and you can read the reviews to find out which ones are good. From a brief scan, it looks to me as if the Visco and ViscO2 get the best reviews.
Once I do a search on Amazon, I usually choose a department and then sort the results by average customer review. That typically gets me the top 5 selections or so to research for myself.
Here's a link to one that I found on Amazon:
http://www.amazon.com/Density-Elasti.../dp/B004AR5X92