I expect to include one at random (whichever ones I have lots of extras for) at no additional cost, but if you have a specific one you prefer please order it from the shop.
Matthew Barszcz
commented
I don’t see a way to purchase the LED overlay separately, or choose any variations of them. I just see the LED Overlay ($5) as an add on option for the Classic Style F104.
admin
commented
It is on the Beam Spring Extras page.

Only the 104 and 122 keyboards have LEDs built in but there are various mod projects to add your own LEDs to any new Model F or new Beam Spring keyboard. Please read the other Q&A’s for more details.