For "rubber feet" my search engine tuned up www.rubberfeet.co.uk at the top of the page. They seem to go up to 25mm - although they are wholesalers and may not be minded to point you in the direction of a retailer.
You can get specific feet that raise furniture by the sort of amount you are looking at.
A friend of mine is unfortunately not well and due to reduced mobility uses a set of blocks that raise his sofa by a couple of inches. Theses are designed exactly for this purpose so are secure, but are not permenantly fixed to the sofa. It makes it a lot easier for him.
You might want to try one of the mobility shops/websites