I have 3 platters that I use for brim blocks. One is round and 13 inches (36 cm)one is oval shaped 15 inches X 18 inches (38 cm X 45 cm) and one great BIG one that is 24 inches in diameter. (60 cm). There is a brim block I saw that I would like very much but it's way too expensive for me. These platters work well for now. You can always make the brim bigger than you want and cut it down.
I’ve purchased several blocks from eBay which were affordable and new. eBay sellers also list used blocks which are economical. I use to buy from a local craftsman who specialized in hat blocks, but recently passed. There are also antique shops and flea markets that ocassionaly sell hat-blocks. I’ve also used items around the house for blocking which worked perfectly.