It turns out these connectors follow a naming convention, e.g. SNB1.25-6, OT2-4, UT1.5-3, etc.
SNB, OT, UT describe the kind of connector (fork vs ring, both SNB and UT appear to be forks, OT appears to be a ring).
The first number describes the diameter of the crimp opening (e.g. 1.25 mm).
The second number describes the fork / ring opening clearance (-6 is 6.2 mm, about 1/4 inch).
You can find these by the thousand pack for under $10 on alibaba and toaboa, but at those prices they are likely to be brass rather than copper.
I find it hilarious that unplated copper connectors sell at a crazy premium compared to tin-plated-copper connectors. In theory, they should be cheaper (one less manufacturing step).
The size we want (to accommodate a pomona 3770) is the 1/4 inch stud (-6), and for crimping to CAT5 / telephone wire, I think you'd ideally want a 1mm crimp opening, so you're looking at either SNB1-6 or UT1-6. Domestically, the closest I've been able to find is SNB1.25-6 (this is the size of the Morris #11520 connectors).
I have seen UT1-6 available on taobao, but copper vs. brass is going to be luck of the draw there, and minimum quantity will likely be a bag of 1000.