Go to ebay and do a search, like "forest bases 25mm round". Or try thingverse and etsy. These are made by a myriad of little companies and persons, and they come and go. Some 3d designers sell printing rights, so you might get them ready printed nearby.
I've got some nice 20mm squares, could be from early Scibor. They make many types, don't know if they make exactly what you want.
-Z