Where can I buy numbing cream for SMP?

You cannot buy lidocaine in general.  There may be very weak formulations on the Internet.  You should consult with your doctor.

Lidocaine numbing cream/foam seems to work with SMP patients. However, it is only available through medical suppliers only.

Numbing cream coontaining 0.5% Xylocaine mixed with 0.25% Marcaine can be purchased through medical suppliers such as McKesson, but you must have an account with them