Digital streaming
All tracks from the albums Shinryoku no Eden, Invert, Piano Collection I, Garden of Vacancy, Karma, and Rebellion of the Enchained Birds are available on commercial streaming services as part of the Metomate Collection I — an anthology released on 2022-12-24, containing remastered versions of all included songs. (In addition, the Shinryoku no Eden tracks have been completely rerecorded with Meto's own vocals.)
The newer albums are unfortunately not yet available on streaming services and must be purchased either digitally or physically (see below).
A few tracks also have official digital streaming releases independently of the Metomate Collection I — these are the original, non-remastered versions:
The situation is only slightly different for Meto's works outside of Metomate. Her higher-profile commercial stuff (rhythm game tracks, UGUISUKAGURA theme songs) tends to have more or less official YouTube uploads. The Tenkahyakken -Zan- soundtrack is also officially available on streaming services, as are the one-off original songs Mei to Mei and Usetsu no Waltz.
The situation varies a lot with Meto's doujin guest songs. A few of these are on YouTube, such as Akenai Yoru, in the world, and Call your name. In cases where Meto contributed to an individual song that stands on its own (e.g. theme songs), it will also often have an official upload somewhere. Her contributions to doujin albums, however, are usually not available for streaming anywhere.
Digital purchases
All Metomate albums (containing the original versions of songs) can be purchased as digital downloads on Metomate's BOOTH. The prices vary between 400 and 1600 yen, and all songs are available as FLAC or mp3, with a lyrics sheet included.
This also applies to some, but not all, doujin albums she has made guest appearances on; I recommend checking the respective circles' BOOTHs for more information.
Physical purchases
For the full Metomate experience — namely the beautiful booklet art accompanying each of the songs — you will have to buy the physical CDs.
The later albums, starting from Rebellion of the enchained birds, are available to buy from Metomate's BOOTH. Some of them are also available on Akibaoo. The earlier albums, from Shinryoku no Eden to Karma, are out of print. They are sometimes available second hand on sites like Surugaya. For information on how to buy from Japanese online stores in the West, I recommend reading Merami fan's guide on the matter.
Meto does not participate in physical doujin events for the time being, so buying her albums directly at M3 or Comiket is currently not an option.