Can you obtain a 45-day tourist visa at the Cambodia-Thailand land border or only by flying?

yes you get 45 days visa excemption with the right passport

There's no such thing as a 45 day tourist visa. All entries with a tourist visa are 60 days regardless of how you enter the country.

If you are referring to visa exempt, meaning you have no visa at all, then that's 45 days regardless of how or where you enter.