There is at least one other manufacturer, but I can't remember the brand name right now. It's right on the tip of my tongue. I think it's a Japanese brand but it might have been a European brand, but it was a smaller brand that mostly rebrands stuff made in Taiwan. It is a 4 letter brand, and I think it might start with "F" or "D". I can picture the adapter in my head and the branding is laser etched onto the adapter instead of being stamped. I think they only made the 3/8" drive version, so Snap-on is likely still the only source for the 1/4" drive adapters.
I tried sourcing the alternative brand in the US a year or two ago, but I couldn't find a website that would ship to the US that also had them in stock. Maybe they are available now. If I remember correctly, the adapters were about $10-16 each depending on the website selling them. If nobody else knows the brand I'm thinking of, it will probably pop into my head within the next couple hours.
EDIT- I just remembered. The brand is "Deen", and it looks like they do make them in both 3/8" and 1/4" drive. If anyone knows of a US delivery option at a good price let me know. Based on how rarely they would be used I could never justify paying the prices Snap-on wants for them.
【Product Specifications】Total length: 31.3mmIt is an adapter that converts the insertion angle of 90 °.By combining with adapters such as handles and extension barsYou can access complex places where work is difficult.
deentools.com
1/4" drive - part number DNBA2Z
3/8" drive - part number DNBA3Z