Our main website is also our point of sale for our brick and mortar and internet stores.
Downtime is a no, no.
Our current Linode is in Dallas. What, if any, mirroring options are there?

Depends what you're trying to protect against.

Generally, I'd recommend using a NodeBalancer, serving your site from multiple app servers behind the NodeBalancer, with multiple DB servers behind that.

That's good enough for most use cases.

The next option up is having a cold or hot standby in another datacenter. Essentially, the same cluster of NodeBalancer -> app servers -> db servers in some other datacenter, ready to fail over to if there's an issue. this doubles cost and adds a lot of technical complexity, given that you need to be able to sync data between the two, and you need a plan for doing DNS failover to the standby location.

To note, it is not possible to eliminate downtime. You can plan for it, and you can work to minimize it, both of which have monetary and technical cost.

- Les