Measure the thread size and try summit racing. They have a big selection of copper or aluminum crush washers. My Flaming River aluminum rack needed special fittings with o-rings on the OD of the fitting, not crush washers.

Fragola Performance Systems AN to Metric Adapter Fittings 491962-BL - Free Shipping on Orders Over $99 at Summit Racing

Fragola Performance Systems AN to Metric Adapter Fittings 491963-BL - Free Shipping on Orders Over $99 at Summit Racing


Yours looks to be machined flat, so a crush washer fitting may be the correct one to use.

Crush Washers - Free Shipping on Orders Over $99 at Summit Racing