Sources for brake calipers

Hard to say without knowing what prices you're getting...
1974 PLYMOUTH DUSTER 5.9L 360cid V8 Caliper | RockAuto
Brake Caliper - 1973 Dodge Dart | O'Reilly Auto Parts (oreillyauto.com)
1974 Plymouth Valiant Brake Caliper | NAPA Auto Parts (napaonline.com)