Since I feel buying and archiving Touhou (and by extension, other doujin series) trials is a largely niche business, I feel it is useful for me to provide my own little guide on how one can purchase and preserve these for themselves. For context, earlier builds of these games are often sold at Japanese events such as Comiket and Reitaisai as sort of special preview discs for the games. There are also many Japanese magazine discs that can include these special trials along with other goodies. This basically makes them betas of the games, often with interesting differences from the final game that are great to document--and, of course, with a cool disc to add to a collection. Downloadable trials include usually little to no differences from the final game compared to these (unless it's one of the rare cases where the disc trial is the same as the download trial because of only one being released, such as with Touhou 13).

As a side note, if you manage to get a trial that doesn't seem to be uploaded anywhere on the Internet, it would be appreciated if you could upload them (and images/scans of the disc(s) if possible) to a service like archive.org so that they can stay safe and accessible to everyone. People interested in playing them (like me) will thank you. ;)

You can find trials by searching from resellers on Japanese shopping sites (like Mercari Japan and Yahoo! Auctions Japan) and purchase them by using proxy services (such as Buyee and ZenMarket) so that they can be shipped to countries outside Japan. Searching for the game’s Japanese title (for example, “東方花映塚” for Touhou 9) along with “体験版” (the Japanese term for “trial version”) shows any listings that are available for that game’s initial trial disc! You can also search "東方体験版" as a catchall for all Touhou game trials (just keep in mind some fan game trials will also come up). In the case of Touhou, they can also often be updated to later trial versions using the updaters on found on ZUN’s sites (such as http://www16.big.or.jp/~zun/).

Happy trial hunting!