The ONLY company that currently makes arms that will fit an Airstream is Zip-Dee. They will probably sell you just a set of arms and mounting hardware, as they often sell replacement arms that have been damaged and need to be replaced.
Give them a call and tell them what you're doing and what you need. They are nice people to do business with.
Solar Tech Energy Systems, Inc.
Victron Solar Components and Inverters, Zamp Solar Panels, LiFeBlue and Battle Born Lithium Batteries, Lifeline AGM Batteries