Auto parts stores, Marine supply, or trailer places usually stock the ones that slide on over the hub. The hubs on my trailer have the zerk that goes through into the inner bearing. This uses a rubber insert that goes inside the hub. These are harder to find. I lost 1 a couple of years ago and couldn't find them so I bought a cheap set of bearing buddies. Discarded the spring & grease fitting - just used the stainless hub part. Hammered it into my hub and now use the external caps. If its a hot day go around and push the center in to release air. Otherwise I think when heat builds up that can be what blows them off.
Rob