Certain drills, such as side kicking and one arm freestyle, demand the swimmer to wear fins in order to maintain propulsion. With the added surface area that the fins provide, the swimmer is able to produce greater propulsion and keep the legs lifted, allowing the focus to be on the drill as opposed to the kick itself. There are endless styles of fins, and I recommend a short or medium blade fin that is designed specifically for swimming, as opposed to diving or body surfing.
The initial purchase of these tools should last you quite a long time. Try to keep your budget under $200, and remember how inexpensive your swim kit is relative to that of your bike purchases. This is an investment into your aquatic future and one that comes with great returns.