Green BMS is an open source project, so you can download the gerber files from download page and you can buy the cards wherever you want.
Alternatively you can purchase Green BMS components on PCBway Store.
For each component there is the possibility to buy only the PCB by carrying out the do-it-yourself assembly, or ask PCBway for a quote to receive the board already assembled, using the BOM file included in the product page.
If you are not yet subscribed to PCBway, you can do it at my invitation link: https://www.pcbway.com/setinvite.aspx?inviteid=436793
These are the pages where you can buy the PCB's: