I have a range of rubber typewriter feet for sale on my website.  This link will take you to the relevant section :  http://thetypewriterman.co.uk/supplies.html   Makes and models covered include Underwood 3, 4, 5 and 6, Corona Four, Continental Standard, Adler Gabriele, Imperial Good Companion, Torpedo/Blue Bird, Pre-War Remington Portable and others.  I also sell the special recessed carry case feet for the Imperial Good Companion (which may also fit other makes).  In the majority of cases these are either hand machined in small batches from solid rubber, or moulded in small production runs.  Although not particularly cheap, these are made from material equal to, or better than the originals by respected suppliers.  Worldwide shipping is not a problem.  A small packet will reach America by airmail within ten days, for example.  I accept PayPal payments !