https://www.charliefoxtrotmodels.com/collections/movement-trays
https://wargamesbuildings.co.uk/epages/950003459.sf/en_GB/?ObjectPath=/Shops/950003459/Categories/Movement_Trays/Irregular_Movement_Trays
However, it is easy to make your own if you allready own a powerdrill, saw and a file.
Find a general hobby shop that sells either plastic or
MDF plates, about 2-3
mm thick, all depending on your needs.
Then you need to find a hardwarestore that sells something called a holesaw. they are basicly circular saws on a drill bit. Buy thouse that matches the base diameter.
Cut the material plate to your needed movment tray size. place out your round bases and draw their shape on the plate and cut them out with the wholesaw. then cut a new plate matcing size and glue the two plates. Shape the edges with the file.
IF you are not a "tools person" there are from time to time makers on sites like Etsy, who makes and sells custom
MDF miniature bases and movment trays.
(or, if you know someone who owns a 3D printer, you can print your own movment trays. many templates out there for that)