The smallest buttons I have found of arcade quality are the Sanwa and Seimitsu 24mm ones. The key phrase being "arcade quality". You can get smaller buttons (around 18mm) from both suppliers but they list them as test switches and they were never made to be used on arcade panels. Because they are test switches they are usually only available in about two choices of colour. How they feel and operate I do not know because I've never tried them.
As far as the Happs style buttons go I must admit I have never seen smaller versions of that design. I would definitely have considered them if I knew they were available.