Hi Tom,
Trying to help, but I also do not want to accidently get you into an issue not knowing how the fastener is being used.
You mentioned, yours has a "hex head" and is a "set screw", that combo of hex head and setscrew is not common. It is not be impossible, but not common, and I have not seen it used with a hex head on Reese or Drawtite hitch.
Which part on the hitch is this fastener used on? And did you buy this hitch new or was it used? Meaning someone may have substituted something. They so make 1/2"-13 full threaded bolts 3 1/2" long, but they are bolts and not setscrews. Most times, they are used in different applications.
The 1/2"-13 x 3 1/2" long. square head set screw is common on the Drawtite and Reese chain snapup brackets or the HP dual cam frame mounting plates.
Here is a pic of the snapup with the setscrew.
The DC frame plate, you can see it in this pic
Sorry for this being this hard, but don't want to suggest a wrong strength fastener in the wrong application. Set screws can be made from case hardened steel and made to ASTM B18.6.2 standards for compressive loads "only". They have a tip on them to pinch hold something from moving, like a cup point etc. They are not rated for tension holding, only compression holding.
If you want a "bolt" they are rated in both compressive and tension loads and strength rated by grade. Grade 2, 5 and 8 are the most common in US standards. These come in different head configurations. These are very common to get and most have some type of plating on them, but they do not have case hardened cup points like a set screw.
Here are options for a setscrew, assuming this is what you need. The issue is to get the zinc coating on them so they do not rust.
I found a Reese hardware kit cheaper then Amazon with the "set screws" in it cheaper then. This may be what you found. They are plated at least.
https://www.etrailer.com/Accessories...ese/66014.html
Both the McMaster Carr, Fastenal and MSC have 3 1/2" lg set screws in plain steel, not coated. Fastenal does not offer zinc in 3 1/2" length. They do in 3" or 4" if that would work for you. These are made to that ASTM B18.6.2 compressive loads standard.
https://www.mcmaster.com/1%2f2-13-sq...ivalent~0-500/
https://www.fastenal.com/product/fas...egation=Length
MSC has them, but they called out a grade 2 fastener which is not very strong, they to not list the ASTM rating. And they are not plated either.
https://www.mscdirect.com/product/details/67391045
The 3 places above are some of the many industrial supply places I have bought from over the years that sell fasteners. They are cheaper, just you have to add freight, but are not zinc coated. Or paint/treat them once installed to not rust
You may try at a "good" old fashion hardware store, or a fastener supply house in your area.
Hope this helps
John
PS. If you want "bolts" they are a easier to find due to being more popular in the 1/2"-13 length.