Where can I buy or get hard drive trays for a ReadyNAS RN104 (RN10400)?

Replacement trays were never widely available and were quite expensive where they were sold (>$50 each).  Unfortunately, businesses get lazy and remove NAS drives for destruction without removing them from the trays.  They have no idea how much that devalues a ReadyNAS.  I do hope you didn't pay a lot for that 104, because the value of one with trays is not a lot more than the typical cost of 4 trays.

The issue I see with that one is that it attempts to clone the second-generation legacy tray but the hook that holds the tray in is plastic instead of metal.  I can't see that lasting.  Some similar ones show making that part from metal.  Cloning the newer one, which has a much wider plastic lip that holds it in, would have been a lot smarter.  There used to be a seller on eBay that sold 3d printed tray replacements that didn't even try to have a latch, but I no longer see a listing for them.  Unless you are moving the NAS with drives installed (a really bad idea), you shouldn't need latches.