The Copplestone train is beautiful, but not really what was used in Latvia.
I game the period in 15mm, so no idea on models.
Have you read "The Outlaws" by von Salomon? I found it very hard to follow, but it does give a perspective on the German side.
"Cowan’s War: the Story of British Naval Operations in the Baltic, 1918 - 1920" by Geoffrey Bennett is a great book, which covers the Latvian, Estonian and Russian operations from a naval perspective. Republished recently as "Freeing the Baltic".
Then there is "L'Aventure Allemande en Lettonie" by Lieutenant General Du Parquet. I translated most of this into English, if you are interested.
After that you are into the native languages I'm afraid.