I bought some of these from a company in America on ebay. Easy to fit to any surfboard or paddleboard and you can lock it to your car or a solid fence with the cable provided.
I've also put all my boards on my home contents insurance as portable items, which means that they are covered if they are stolen and I have kept a note of the serial numbers of all the boards and recommend everyone does the same.
Luckily the community is still quite small so it's going to be quite hard to offload stolen gear without someone finding out.
Good luck getting the board back.