Happened the other way around. Your bearings were bad, and when they got enough slop in them, the spindle nut knocked the cap off.
In other words, with good bearings, you can actually run for a very long time without the dust cap. The crud getting in there will destroy the bearings but it takes a while.
Dorman doesn't make a direct replacement but you can try Dorman 618-102, it's the one with the small domed top. You will likely need to use a punch to dimple the edge of it in several spots to make it stay on. On the upside, I've used this cap to clear "problem" custom wheels that have a center cap that doesn't have enough clearance otherwise.
Or hit the boneyard and hope for the best. Being you're in DFW area you should be able to find plenty though there's not near the amount around ever since Cash for Clunkers.
Richard