As I posted above, it looks as if SFFLAB are out of stock of panel kits and don't plan to restock them. There's apparently a new version of the M1 coming this year (v6) but it won't maintain compatibility with previous versions when it comes to panels, so it looks like second-hand is going to be your only option for panel kits for v5 and earlier.
Sorry I should have read the whole page.
I guess if the V6 is a decent upgrade I'll be getting that plus its corresponding replacement panels.