Is it possible to get a longer power cord for my iMac 21.5" 2019?

The power cable uses a standard IEC 320 C13 connector commonly found on many consumer electronic devices.

You can also use what's commonly known as an "extension cord" but if you're looking for the molded white variety that fits flush with the back of your iMac the answer is no.

Just to add, Surge Protectors often come with longer cords so you could plug your Mac into one. Using a surge protector is not a bad idea to protect your Mac.