Rick,
The butt plates and other M2 parts are occasionally offered on Ebay but there are some dealers on the M14icon Forum and a WTB ad will probably yield some results. Also, try GunBroker. The sights on the many M2's I've had have all been 48C's. Early in the M2 series they went to an adjustable headspacing bolt and those already in service were so modified. $150 for an original mag is about typical. Definitely try the M14 Forum because several of their sponsors trade antique US military parts. Good luck!
OC