There is a significant overlap between the categories “router bits” and “end mills”, something Amazon reflects by freely switching between the two in search results, so that it can be a bit tricky to find the exact tool you’re looking for. In almost all ways, a search that begins by drilling down (sorry) through the category path Industrial & Scientific → Cutting Tools → Milling Tools → End Mills will be far more useful than a top-level search that inevitably takes you to the Router Bits section of the Tools & Home Improvement category.
The reward for careful navigation is the ability to refine your search by almost every possible characteristic of an end mill: material, coating, overall length, end cut type, cut type, flute count, and cutting diameter. If they added shank diameter, it would be perfect.
(still don’t have my Shapeoko 3 yet, sigh; according to the last update, there was One Last Part that Finally Shipped, and once it arrived and had some finish work done, they’d start charging credit cards and shipping out product)