Most sets have the common size. HARBOR FREIGHT has Pittsburgh brand and have a very good fit. Here's the kicker. They include 3.5 mm and 1.27mm. 1. 27 is hard to find. Thats is the size to adjust Impact (among others) trigger blade.
Just be careful because they snap easily (especially the smaller sizes). I snapped one off in the shroud set screw for my Taipan Vet. and it was hell trying to get it out without damaging the shroud. The screw wasn't even that tight as I had previously installed it and the ball still snapped right off.
Have a friend that used to be a mechanic. His theory was that you bought all of your tools at Harbor Freight and used them until something broke, like an individual socket. Then you bought the SnapOn version of the socket because it was something you used a lot. At harbor Freight prices, you can afford to replace fairly often as well.
I have exactly the allen wrench set you are talking about, and it works great (though the smaller can twist really easily!)
I pass a Harbor Freight on the way home from work. You can guess that I have lots and lots of Harbor Freight tools. Some have been good, some trash out of the box, but more good than bad.
Yet another addiction to airgunning, tools to make it all happen.