If your local fish market sells frozen tuna steaks those should be okay. The commercial/deep freezers are supposedly more safe as they have a more stable temperature. And IIRC they need to be frozen for at least 72 hours.
Also, make sure it’s flash frozen. I believe it says so on the packaging.
On top of that find a vinegar or salt recipe to add flavor. I believe it needs to be somewhat marinaded but I’m not sure the sushi-stores do so.
If it seems too complicated there’s always frozen imitation kani.