One of the universal holsters with the velcro adjustment is your best bet. Or you might try thingverse for a 3d printable one. There was a shoulder holster rig but haven't seen one for sale for a while.
and there is a fabric one made by Leapers/UTG but I think it may have been discontinued. As mentioned above one of the universal adjustable ones may be your only bet, alternatively you could sling it and use a weapon clip to stow it on your belt.