I have two Mega Drive themed controllers from Retrobit, their official names are:
Mega Drive 6 button Arcade Pad with USB
and
Mega Drive 8 button Arcade Pad with USB
Both are wired controllers with USB connection.
I tried them in the A600GS, and they work perfectly, in exactly the same way the controller that comes with the GS does.
Both controllers appear on the Controller Preferences screen as "SWITCH CO.,LTD. USB Gamepad" (see attached picture)
Buttons A&B work as buttons 1&2 on the Amiga
The Mode button brings up the on screen keyboard
And the Start button brings up the F12 menu, in exactly the way the GS controller's start button does.
(The two extra shoulder buttons on the 8 button pad don't offer any extra functionality over the 6 button pad on the GS)
These two pads are a perfect alternative to the official A600GS controller.
Both pads come in both Mega Drive and US Genesis themes and different colours.
Pictures of the 6 button pad in both Mega Drive and Genesis themes attached
(I have the Clear Blue Megadrive themed one, as in 2nd attached picture below).
Retrobit official listings:
https://retro-bit.com/sega-genesis-6-button-arcade-pad-with-usb-clear-blue.html8 Button:
https://retro-bit.com/sega-genesis-8-button-arcade-pad-with-usb-clear-blue.htmlThe controllers are also available on Ebay and Amazon in both Mega Drive and Genesis themes (prices vary).
I also tried an original 6 button Mega Drive controller in the DB9 port, it works with A&B buttons as Amiga buttons 1&2.