Well it's a common size that can usually be found in a jar or can of old spare/discarded nuts and bolts that any well equipped shop usually has under some bench somewhere...
In lieu of that I'd suggest grabbing an assortment #6,#8,#10,and 1/4" in both fine and coarse threads and then another batch for 5/16 through 1/2".(cause you'll be needing one or another at some point for something anyway)
When I have that question and don't feel like looking through my jars and muffin pans, I grab the tap and die kit and figure it out that way.

Give me ambiguity
or give me something else