Are you possibly referring to the much smaller pin which is issued only to CoS Agents? If so it is not available for purchase, but only by issuance from High Priest Gilmore himself.
Perhaps it's not the one you mean, but if it is, I thought I'd make you aware that this very small lapel pin is not one you can buy.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."