[sdfem17]

Lots of decisions to make to select the right pair of swim fins. If your swimmer is on a club then I assume he or she is working out in a pool. You need a different type of fins for the ocean....
Find out what style and what size your swimmer needs. Then try e-bay. I just ran "swim fins" and lots of inexpensive models were available. It does not matter in the least if they are used.
Also you might try a used sporting goods store. Or if you live on the west coast, just check out your saturday morning garage sales!!
Good Luck to you and to your swimmer! It is a great sport - and the only one that can save your life!!! Kowing how to swim also opens up a world of job opportunites later in life.