You can find compatible casters here.
Fasteners for newer BB15s: M8-1.25 x 25mm Button Head Hex Socket Cap Screws
Fasteners for older BB15s: M6 x 25mm Button Head Hex Socket Cap Screws
Newer versions of the BB15-MK3 and DJ1818S-MK3 models have installed hardware to accept caster wheels.
With caster hardware:
BB15-MK3 SKU: Ends in BB15-MK3
DJ18S-MK3: SKU: Ends in DJ18S-MK3
Without caster hardware: