I honestly don't think he would want to get another arm. He makes it abundantly clear he feels a great deal of guilt for what happened to Nanamo, and the man is filled with a great deal of pride. It would make sense that he treats the loss of his arm as punishment for his failure to protect his charge.

Besides, I think it's far, far more awesome to have a guy who can absolutely decimate nearly anyone with just a single arm (see Auron, who has his arm on a sling for no reason most of the time).