Hi Herb,
my impression is that this item seems to come available briefly, then get sold out; that may repeat or not. I'm not passing judgement, I'm simply reporting what I found.
Being one person, I only sell what I've assembled or put together as a kit. I don't like to sit on folk's money while I work and prefer to ship things immediately. We all know many things in life can come up unexpectedly. I think I see one such dongle on this device, on a board that probably
includes another processor and some RS-232 type level-converting. One could likely build one with an Arduino - some of them are WiFi without a shield. For fun, you could add a speaker to sound-out the wired-modem buzz-whirr connect sounds.
This board does indeed use a ESP8266 WiFi microcontroller and a RS-232 level shifter plus the "modem" firmware that I wrote to "glue" the pieces together for your vintage computer.
One challenge of vintage use of 21st century connection technology - WiFi, even USB/serial dongles - is how they manage both sides of the 20th century transfer. Translation: USB dongles have hardware buffers (and software drivers) that screw up character processing and other delays, by old MS-DOS programs and old slow microprocessors. I don't know if the WiFi dongles have similar issues.
This is certainly still a potential issue. Latency introduced over WiFi will cause problems for things that rely on hard-wired connections to be nearly instantaneous. I've also seen the same buffering issues you describe with USB to RS-232 adapters. Some buffer a much larger amount than others and seems to vary widely. Cheers, -p