Was at home depot. They have that bolt but not the button head.
Check the "Specialty" fasteners drawers at HD... they have stainless button heads, not certain of thread pitch available for 3/8".
If there's a Lowe's nearby check there... I know they now carry button heads also.
Pretty much tinker with
everything and doing alterations on motorcycles as often as I do I'm frequently looking for special bolts and machine screws not found in the usual places. Often need to alter the fasteners as well.
Besides the above big-box stores and my own local 'creaky floored' hardware vendor there's an Ace Hdwe. I haunt and three different industrial supply outlets in the vicinity that know my face by now.
Check your local Yellow Pages under "Bolts and Nuts" (not the other way around) when you come up empty for a specific item elsewhere.