We currently use NService bus with a distributor but I'd like to make it HA without moving to clustered MSMQs. We already have a clustered SQL server. Is there a way to have NService bus Distributor use the DB instead of the MSMQs
Since the MSMQ service is what is actually receiving the messages, unless it is highly available as well, it's not enough for just HA for the distributor. This can be achieved using fault-tolerant hardware for better failover time than with a cluster. If you want to use the database, you'd need to implement a DB transport for NServiceBus and recompile the distributor - but that still wouldn't give you HA for the distributor as you'd need something to failover the distributor process.
Have you considered deploying the distributor to the cluster that is running your SQL Server? That might be the simplest solution for you.