Where can I find nice knurled thumb bolts for a rack?

Try searching for rack thumb screws as well as thumb bolts. Hammond makes rack thumb screws, though the ones mentioned here have a black oxide finish rather than the shiny look you want.

Check the thread and length before ordering a pile of them. A thread pitch gauge and calipers will help you match the diameter and pitch to your rack hardware. Don't rely on the head looking right in a product photo.

Some older Cisco Meraki switch hardware bags appear to include similar bolts. Unopened bags may turn up from resellers, although buying enough individual bags for a whole rack could be a hassle.

If the goal is easy finger tightening rather than matching the finish, look at Rack Studs. They're a different mounting system, not shiny metal bolts, but they avoid handling cage nuts and don't need a screwdriver to tighten.

Check the manufacturer's load rating and compatibility with your rack and equipment before using them for anything heavy.

Another alternative is the Patchbox Dev Mount system. It's designed as an alternative to cage nuts and may be convenient if you're changing equipment often, though it costs more than basic rack hardware.