You will need a whole new RD and chain, you can purchase some part of the RD as spares (the inner plate) but not the front, and even that will cost a significant part of the cost of a new RD (been there). If you were to look at cannibalizing another RD, the question would be why, as you would be stripping a good RD to make another good RD.
Depending on the model you may be able to purchase a complete aftermarket cage, these are normally sold with oversize jockey wheels but can cost 2-3x the cost of a new RD