What is this tap connection/knob called? Can't find it in hardware stores

[zakirit]

I went several hardware stores to find a new cold water knobs, but they all said this is an old connection and don't sell it.
I want to order it online but I have no idea what's the name of the connection and which knobs I should buy?

 

[breplum]

Most pros on this site are USA based, and I do not recognize that (what we call a) valve stem. Chicago Faucet Co in USA uses a square end but what you show is not Chicago brand. Some plumbing shops (call around) have adapters that can work with a variety of old stems, and then a universal handle on the adapter.
Avoid universal handle kits that use two allen screws that clamp down from offset directions. Those slip and wreck the stem end.
Replacing the entire thing is your best investment.