I would look for a junked Cinelli stem at the most old school LBS you have nearby. Or on ebay. Yes, you are buying the whole stem, but you don't have to worry about its extension or the condition of the stem. All you care about is the bolt(s). Should be cheap and available. There are several on ebay right now under "Cinelli quill stem". One isn't being bid on and is going right now for $0.99 There is also a bolt being sold by itself for $25.00. You do the math!
Be careful, the high end ones come with an aluminum bolt. You may or may not want that.