Hi!
Can't find them either... They are hard to find! I can find the actual joysticks, but not the caps.
Maybe this is a solution (if they are still okay-ish..): https://www.ebay.co.uk/itm/364969986842?chn=ps&_ul=GB&_trkparms=ispr%3D1&amdata=enc%3A1BKvRHEwnQquBpyxPQ_VeaQ12&norover=1&mkevt=1&mkrid=710-169684-531343-4&mkcid=2&itemid=364969986842&targetid=325425753764&device=c&mktype=pla&googleloc=9193247&poi=&campaignid=20649934318&mkgroupid=153764631759&rlsatarget=pla-325425753764&abcId=&merchantid=710354707&gad_source=1&gclid=EAIaIQobChMIjavvjarJiAMVJpSDBx2WlDs4EAQYGCABEgJpjPD_BwE