Proto Industrial Tools do a range of pin spanner wrenches (or C spanners if you prefer). A quick look shows that the Proto JC495 seems to almost match your spec. (1/4" pin diameter, 7/32" pin length, 1-1/4" to 3" capacity) - Easy to source from multiple places in the U.S. - MSC Industrial are usually a good source for that kind of thing.
Be aware that a pin size can refer to length or diameter, so when you say ďalso the larger wrenches (i.e. 3") seem to all have pins bigger than 1/8" diameterĒ are you sure you are not looking at length? Are you sure that the 1/8" pin in your diagram refers to diameter? You might have to do a bit of research
but you can also try Armstrong, NuLine, etc. Unless you know that you need the full range of adjustment, it might pay you to measure and find out what exact size(s) you need.
In the U.K., Teng make a really nice set with multiple sizes, ends and pins. Itís pricey and of course metric, but I've used them and they are really nice quality. You can also try Cromwell Tools.
If you are really in Shenzhen, there are some fabulous tool shops both there and in Hong Kong
. The factories that make some well known brands often take the same tools, re-brand them and sell them locally for a fraction of the "western" price
. Not everything that comes out of china
is poor quality. Take a picture with you as translation might be a bit difficult. If you don't know where to go, find a local workshop and ask them where to buy tools. Or stop by Zhuhai on practice day and ask people working on their bikes/cars. There are often a bunch of Hong Kong
based ex-pats there as well, or at least there used to be.
Also, Progressive Suspension make one for adjusting motorcycle suspension, so if there is a motorcycle shop nearby.... I just measured the one in my toolbox and the range is right although the "pin" was difficult to measure, but it's made from flat stock so you can always grind it a bit, if necessary. It's probably made in China
and available under several names.