Caravan shop ... as sometimes you find similar for awnings ... or odds and sods bin at chandlers .....
If that fails - try somewhere like Seateach or any outfit that makes sprayhoods / canopies etc. Telephone them and they'll probably post a few to you.......
AND last but not least ..... you can get something similar in B&Q ... probably black plastic instead of white ---- so change em all ... they are normally wityh the rope and garden shackles etc.
Do you mean the ones with a very thin plastic square section which is machine sewn-through to attach them?
If so. I believe that they were discontinued some years ago due to UV degradation causing them to break off along the stitch line. I replaced mine on the last but one boat with rubber awning toggles and the largest size of Hipkiss eyelets,