Online is best option if you don't have a local computer-builders shop nearby (I expect they'll have a few kicking around from PCs they've built if they haven't tossed them in to a skip). In Google Search, type PC drive bay blanking plate and it'll bring up loads of links. I see Ebay comes up a lot if that interests you.
You may have to search a bit more if your case front is a special design instead of being plain & flat, or if it's aluminium and you want the blanking plates to match it.