You can get these round (and some square) pinback buttons in a range of sizes, from a self-effacing 1 1/4" to an emphatic 6".
The prices shown are for the 2 1/4" size.
Now, granted: you might rather have a penannular brooch. But unless you're gathering your cloak around you these may be a bit more practical.
They won't leave those huge holes in your clothes, for one.
Of course if you really do want a penannular brooch on your shirt - while avoiding the gaping holes problem - you might try this.
Orders for merchandise from 'Ars Celtica' are filled by Zazzle. They're billed and shipped separately from merchandise ordered through the vendors who supply 'Saga Shirts' and my other products.