The USB keyboard extension wire was included in-the-box with the extended keyboard or
the Pro keyboard. I found a great deal on a new-old-stock white Apple USB keyboard like
what shipped with the last eMac & early Intel iMac, at moving sale, in the box with that wire.
Cost me $5. but I had to buy a large sack with a bunch of great as-new stuff for $50. So
that got me an unused Leopard 10.5 retail DVD in box, and some other items plus software.
Probably a few hundred dollars worth of Apple-branded stuff, which certainly made my day.
Some computer stores may have a USB extension cable. A few years ago I got one in a
kit which had three cords plus one extension. •Another idea if you can find one: an adapter.
The adapter fits the end of the standard printer USB cable to turn that into the correct end.
.Not sure where you may buy one now, I found mine in a cord bin 25¢ at second-hand store.
{Just today, I found a good deal on an Apple USB mouse with Roller-ball. Not sure, but it may
be an original Apple Mighty Mouse. It cost me 50¢ and now I have to clean the ball & test it.}
Hopefully by now you've located the USB keyboard extension cable. I have a few that I use
with card readers so I do not have to reach around behind computers to plug stuff into a Mac.
And also use portable Macs as desktop computers on occasion, so extra wires + wi-fi, helps.
Good luck & happy computing! 🙂