You can take a cab to one of the two Best Buys nearby, at Gateway Brooklyn or Green Acres Mall, ~15min ride in either case. If you know what you want, you can purchase in advance online then pick up at the store.

Best Buy near JFK

24 hour luggage storage is available at T4 if for some reason you can't check bags through to your destination beforehand. Alternatively, if you have enough time, check or store your bags and take the AirTrain + LIRR to Penn Station, from where B&H Photo is a block away.