Amazon has PG36 cable glands which should handle 6/3 soow wire. If you look under the YXQ products they list there is a chart that shows the wire size the different size cable glands accommodate.
You might do better searching for an electrical supply distributor in your area. This is not an uncommon item for them. They all have websites these days. GEXPRO, Rexel, perhaps McMaster Carr or Grainger.