How do I hire a seasonal employee now for a job that doesn't start for two months?

You can certainly ask for them to sign an agreement to be available and dedicated to working exclusively for you for tax season, but whether they will abide by the agreement or not is another question.

That is, they can change their minds, for example, if they get a better offer, take a permanent job, or whatever.

An agreement that includes consideration might give you more leverage. For example, pay them an amount in return for their pledge of allegiance. Even then, if they return the money to you, they may decide not to work for you when tax season arrives.