Hi Martin, the screws for the resin top rails are usually #10 x 1" screws. Your best bet is to head to your local home center (Lowe’s, Home Depot, or Ace Hardware) to get them. Bring along a sample to be sure that the size and length are correct.
Your top plates look like they are these 1320178 and your 24’ Round pool should use 16 of these 1470100 slotted tubes. We still have a few liners for 24’ Round Pools in stock at great end-of-season prices so be sure to check them out!