Replace one seal? I only have them in kits, replacing both seals at once.
From memory, no time to investigate: Both rear calipers, complete seals for about $25-$29 + $4 postage. 50-c worth of rubber in a plastic blister pak. Not worth it......unless you NEED it!
MoPar only offers complete piston+seal rebuild kits for Big $$$ for the boots/pistons, so by comparison $29 is cheep.