what size spoke key do i need for DT Swiss Pro Lock Nipples?
also, what spoke keys are worth buying .

cheers

Try a red spokey pro, available via madison so just about any shop should be able to get you one.

I've got a little metal triangular key from Park which does the three most common sizes.
No idea what sizes are on all my wheels so it does the job for everything!

Pedros spoke key for me – has one three sided key and one four sided one.

DT Swiss will be 15g I believe, or at least that's the one I use.