I don't think it is sold separately, contact some ICE shops to see if they have one lying around or can get you one from JBl.It is a better idea to use the sub control on the HU if present on the HU.
Yes its there in HU, but need to go inside menu to control it, this knob is easy to operate, so just looking around found the model number which is discontinued JBL 109JBLRLC.