In the US you are unlikely to get metric threaded legs at a local hardware store.

But yeah, knobs are knobs. From time to time I help assemble scores of tiny Ikea dressers for a charity. About 1 in 6 ends up needing rework because something is either machined poorly or breaks during assembly. The most common failure is split knobs, so I bring a sack of fun hand painted knobs off of Amazon and give the top drawers "accent knobs" on enough units to complete the assembly run. I'm told the accent drawer units are very popular with the end recipients.

Most of my Ikea stuff I end up customizing in some way. Got a giant toy box drawer - cut a big hole in it and it's a perfect litter box enclosure. Hung a Kallax on the wall, and used my own brackets to line up with the studs. Have Besta cabinets in my kitchen and living room, and used my own hardware.