How do I check and/or find a brake proportional valve oem replacement part for a 1999 Lincoln Continental

Proportional valves are designed to give the right amount of hydraulic pressure to the back brake to prevent them locking up with a resultant loss of control of the vehicle
replacement of calipers would only be need once to fix jammed or leaking pistons but the pads may have to be replaced more often.
if it only the front brakes that is the problem ,I suspect that the fault will be in the compensating ports of the master cylinder that is not releasing the fluid back into the reservoir and the resultant heat build up from the pads still in contact with the discs is building up brake pressure and so keeping the pads on the discs to the point where lock up is evident
failed proportioning valves will result in no brake action to the rear wheels or rear wheel lock up
If parts are not available from the manufacturer spares department, they should be still be available from reputable brake shops as car manufacturers do not make brake parts as such but rely on brake manufacturers to supply items to specifications
the unit will be repairable by an accredited shop and failing that would be available from the wreckers.