Unfortunately I don't think there's any other good source for these bikes besides Yahoo. A couple pop up on eBay from time to time, but mostly not. There is no cheap way to get these unless you get very lucky. The one or two I've gotten from Yahoo I bought as a frame and built with parts on hand. Still not very cheap.

Part of the reason for that is that Japanese framebuilders generally don't care to export their frames or open their order sheet to foreign buyers. This means there are not a lot of these used frames outside of Japan because there aren't a lot of new ones going out of Japan. They have enough demand domestically and don't want or need to figure out how to cater to non-Japanese-speakers ordering bikes. It's a tricky process, fulfilling an order correctly for someone overseas who does not speak the language - these bikes are custom and if you can't take the riders measurements or communicate their preferences well, that provides a barrier. A lot of Westerners interested in these frames travel to Japan to order and pick up the bikes.

Builders like Cherubim and Toei have a few bike shops around the world that can help place an order. I know that Cherubim has a "dealer" in Canada and in England, not sure about the US. They are very customer-focused, with a couple of retail shops and English-speakers at the company. They also build hundreds of frames a year (I think mostly for Keirin riders at this point). Similarly, Toei can be ordered from a select few of bike shops outside of Japan. Jitensha Studio in California is one such shop. Kalavinka used to take foreign orders but these overwhelmed their business and so they went back to just domestic orders.

The smaller builders, like Amanda, Alps, Jingane Pegasus, Watanabe, Sannow, Hirose, and some of the Keirin builders like Samson and Bomber Pro don't (or didn't) have the resources to do that.