Loose balls cost only pennies each. Different hubs take different ball sizes and counts. But 3/16" with 10 per side is a common spec for basic bike shop quality bikes for many decades. If you took your ft wheel with the axle and bearing fittings loose enough to remove with your fingers to a shop and asked them to look at the opened up bearing they should be able to give you FAR more specific help. And then sell you the correct balls. There is a chance that the bearing surfaces might be unevenly worn from having too few balls. The bearing surface on the axle, the cones, are in theory replaceable (if you can source them). But the surface in the hub shell , the cup, is usually not replaceable. Don't be surprised if the shop's suggestion is to replace the wheel, if so ask them to show you their reasons. Andy.