You should not need a new servo arm. The servo should come with one but you can get adjustable ones one eBay or Amazon.

Or you can get aluminum but the stock one should be fine.

Aluminum Servo Steering Linkage Tie Rod 46MM for Traxxas 1/5 XMAXX 7747 1/10 MAXX 8947 Blue https://www.amazon.com/dp/B071LTVTSY/ref=cm_sw_r_cp_api_glt_fabc_R8TD1Z1103H0X2NT00V0