I'd like to hear from the OP as to why they think they need a new RD.

A lot of times, when clients have a shifting problem, they think it's because of their shifter, cable, chain, or the RD itself.

The RD can get bent out of alignment easily. One fall of a bicycle onto the drive side is all it takes.

Recently discussed here and how to fix it:
Freewheel derailleur vs. cassette derailleur?