I hope you find them, but the only 24mm buttons I have seen are the SANWA style which on CONVEX instead of CONCAVE
Like these:


If you do find some, please let me know..
The Sanwa buttons are great buttons, but the colors do not match those seen with HAPP buttons (IL) and of course are a diff style with the switch built in. RICHARDP71 has some of theses buttons on sale for $2 or $3 each.
-Goz