the new engines have very hard valve seats which dont wear out that quick. I read that you claim your mechanic adjusted the tappets, If that was done by "mechanic removing the head and shipping it to lathe shop" then the shop ground the valve stem tips and screwed them up.
The tappet gaps are set by replacing the "lifters" - toyota make 11 different sizes of these lifters. Setting the tappets is a bit expensive business on these new engines.
For new valves - buy them from a toyota parts supplier for replacements or from the dealership/showroom for genuine toyota items.