Fin retainers are also known as fin grips or fin keep(er)s. Their original French name, fixe-palmes, is a tribute to the Marseilles-based diving equipment company Beuchat which patented the idea back in the early 1960s and which also devised the original Jet Fin.
Fin grips are designed to keep full-foot, not open-heel, fins attached to the feet in strong currents. They generally come in three sizes, Small, Medium and Large. Be aware that the Large version isn't that big, so if you are wearing bulky drysuit boots under your full-foot fins, the fin grip straps may stretch, making a very tight, circulation-restricting fit. Try the boots, fins and grips together at home before you use the ensemble in open water.
Here is an image showing how the triangular straps are worn over the instep, arch and heel of each foot and how many colours are available. The range of colours is popular with underwater hockey players who need fin grips to secure their fins when they play in swimming pools.
Click on the thumbnail to see the full-size version:
Such coloured fin grips can be purchased online from the
French Underwater Hockey Association at the cost of 10 euros a pair.
If you just want plain black fin grips, there are indeed several North American online retail outlets selling them, e.g.:
Fin Grips - Scuba Gear & Diving Accessories
Fin Grips