I would think that any Sunfish dealer should have them or at least can get them for you. If you are not near one then I suggest Annapolis Performance Sailing on the internet. (Just click on their ad banner at the top of this page.) APS is where I got my set.
Why not just use line? It is much cheaper than clips and easier to come by. I hear that 1/8" nylon venetian blind cord from the hardware store cut to 12" strips with a tiny figure 8 in each end (second one tied after it is through the sail grommet) in will just barely tie a square not around the spar that you can then pull tight by pulling the grommet just a little, works good for me.
Option 2. If you are not going to race, consider 1/4" wide nylon ratchet cable ties, cheap, strong and easy to install. Just insert through the a grommet, wrap around the boom or gaff and secure with ratchet, trim excess. Don't install too tight - leave about 3/8" between the edge of the sail and the boom/gaff.