They also call this “furring” strip. It comes as 2″x2″ and 1″x2″.
Of couse the actual dimension is really 1.5″ x 1.5″ and .75″ x 1.5″ respectively.
If you have a good “heavy duty” table saw, you could also save some money by ripping a 2″x4″ down to the required size.
Because it’s a good way to loose a finger or two, I only recommend this for experienced wood workers who know how to safely “rip” on the saw. Always use small wood guide block to keep your fingers away from the saw blade.
😆