You are looking for what's commonly referred to as "Foam Rubber". No, there's no rubber in it, but it stretches and compressess like rubber and returns to it's default shape, and is full of air holes, thus the name, Foam Rubber. Usually available in cream or charcoal colors.
You will need to look for "Upholstry" stores. You know.. the kind of stores that sell all the stuff for you to redo furniture. Foam rubber is commonly used in chair bases and backs and other things. These stores also usally sell leather and vinyl on cloth that are used to resurface old chairs/etc..
In most areas you will usually have some time of "all around" hardware store that sells seconds and things like screws and bolts individually (or by the #) instead of pre-packaged, and these places usally have a section dedicated to upholstry and often have stuff such as old office furniture and even boat and car seats! (Yes, in the 3 cities with 50,000 or more people within 200 miles of me, each of them has at least 2 places like this that I am aware of)
Hope this helps!