Can you help me find the part number for this push button cap?
It looks like you have to order them from an Omron supplier, and I'm not sure they've got caps like you want.  I find many different caps that will fit your switch at Mouser, and I'm pretty sure you can get them from other suppliers as well.  Whether you like Omron's caps is another question.

This is a link to the Omron datasheet for the caps.

If the Omron caps aren't what you want, then you'll need to look for a dealer who sells Amico parts - that's who made the ones on Amazon.  I'm not finding them, though.  

Assuming the Amico caps really fit the Omron switches, maybe someone else's caps will fit as well.
E-Switch has caps much like the ones on Amazon.  Maybe they also have switches like you want, and caps as well.  E-Switch seems to have an enormous selection of caps.