You can find the KH 4 piston calipers(rebuilt) from numerous sources, or you can do it yourself with the parts available.
Try Woody's for rebuilt calipers (the link is for rotors but they also have KH calipers:
http://www.woodys-auto-supply.com/auto-part-details.asp?prod=WAG-BD60123Also try
www.brakewarehouse.comRock Auto was already mentioned; Look up a 1970 Dart if you initially strike out; Look for the A1 Cardone rebuilt calipers (loaded with pads or not). Part #s ==> 131.18-4406/7 (left or right)
If you want perfection, i.e. lifetime stainless steel bore inserts, etc. you can get your calipers rebuilt by (but be prepared to PAY!) :
www.calipersonline.comThere are MANY other rebuild services available on various websites spread all over the web.
There are other Mopar websites that specialize in A bodies, and have parts archives, do a search, for something like
www.forabodiesonly.comBest of luck with it!