I have been searching for blind nuts for a use I have that is outside model plane use, although I do build and fly RC planes. But for this use, I need t-nuts that are longer than what I've been seeing. At least 1/4 inch long would be nice. I really need to use 1/8 (#5) screws, but that search looks hopeless. So I can get by with 4-40, which seems to be 7/64 screw size. But all the blind tee nuts i've been seeing are short, maybe only 1/8 inch long. See link for example. Thank you.
The link I tried to post will not go through. It is a Du-Bro SKU: 135

If you haven't already, I would check with Micro Fasteners. They might not have what you want, but they might be able to suggest
a source, since they probably buy from many different suppliers. You also should check with Macmaster Carr.

You may be able to tap a 4:40 out to a 5:40 if you can find something close. Or use a rivet? Not knowing the application.

www.mcmcmaster.com has just about any hardware you can dream up and some that you haven't heard or thought of

Since I don't know what you're going to be using the "blind nuts" for, I figured I'd throw out something a bit larger and used in commercial aviation:
https://www.clickbond.com/products/studs-standoffs
These can be used for surface mounted or through panel installs. Yes, they are spendy but it's another option

You might also consider threaded brass inserts that sit flush in the hole.

I used to get the longer 4-40 t-nuts at an ACE hardware or True Value in their racks of pull out boxes of odd ball hardware. I was usually pissed because I wanted the shorter ones. A lot of them are doing away with the real small nuts/bolts/etc that we use on models but if you find a mom and pop store they may still carry them.

A mom and pop store in Charlotte NC? A city obsessed with high rise diversity developments of imported urchins who know not yet toilet paper? Ace Hardware? True Value? Maybe some place somewhere, but not here. The biggest market here for hardware-related purchases is leaf blower dealers and paint stores..

I've found that Micromark has a lot of the smaller stuff, right down to to downright tiny when it comes to screws, nuts and such. The only issue is that what they carry is brass with slotted or hex drive heads. Here's a link to their fall catalog:
Micro-Mark Early Fall 2021 Page 116

Actually, there are two ACE hardware stores within 5 miles of Charlotte and 3 True Values within 30 miles.