The DIN (German) standard for a M10 bolt is 17mm hex which is what I find in most
wrench sets, but the ISO (international Standards Organization) is M16 which for some reason is hard to find.
So THAT'S where the 16mm head sizes on 10mm bolts comes from. Yes, indeed, a rare size in the US.
I just love Toyotas, where all you need is 10, 12, 14, and 17mm wrenches for just about everything.
Fords, OTOH, often have a hodge-podge mix of metric and SAE, and require more than 10 different
wrenches.
To the OP: I think Sears and O'Reillys Auto are OK to go to for one-at-a-time wrench purchases. HF is
OK for sets, no problem. I have 3/4" metric sockets form HF, and they are fine....my 3/4" SAEs are
old Proto.
For extra-large metric combo wrenches (over 24mm), I now have a set of Chinese crowfoots, which
are fine for the very limited times that I need them.