Herb, as a salesman for a fastener company, finding those wholesale - while not impossible - will prove to be difficult. Most threaded inserts are designed/manufactured for the smaller thread sizes. When you get into the 3/8-16 thread that the bottle stoppers use, most other fastener connections require a stronger connection. A knurled knock-in or mold-in insert as pictured is just not going to cut it most of the time. I would suspect that either WoodnWhimsies, or their supplier, had these specifically made.
However, all that being said, we do offer a threaded insert on our company website that should work. At the end of the day, for a bottle stopper, there's not going to be much difference or selling feature for any one in particular. This insert is a screw in, rather than mold/knock in, and would work best on the wooden stopper blanks since the external thread is a lag type thread. However, we do have customers who use these in plastic applications. The pilot hole we call out for this insert ranges from 5/8 - 21/32. To be safe, and to prevent the block from splitting, I would lean toward the higher end of that range when making a pilot hole. Since this is a screw in type insert, no glue/epoxy is needed.
I've included a link to this product on our website below. 10 pcs is the min and you're looking at roughly $0.50 each and we have both plain steel and zinc plated in stock. Zinc plated is currently showing out of stock but I'll get that fixed with IT.
For the record, the website defaults to UPS and the charges are estimated from our LA warehouse to make sure we pre-authorize enough on your card to capture the correct amount. Honestly, UPS charges will kill any order you want - it's high. You can put in a note though to ship USPS flat rate. The website will still estimate UPS, and the pre-authorization will still be at the UPS amount, but we will only capture the parts cost plus $6.80 (or higher for larger flat rates) when the card is finalized.
PM me and I'll be glad to send you a sample to test if interested. **EDIT - beginning tomorrow (1/13) I'll be on vacation out of the country so keep that in mind if I don't respond right away. Glad to answer anyone's questions about these as well - just keep in mind my schedule. I'll be back stateside on 1/21 and can answer PMs then.**
Threaded insert link