Hopefully not late...
The main thing to know is that any authentic creepers have a crepe sole, whereas fashion versions will most likely have a rubber sole, and there are benefits to both. Crepe soles are unusually heavy, but spongier/bouncier and more durable than rubber, and the material is sort of sticky or tacky (it'll really pick up animal hair for example). Finally, crepe is a much more expensive material, so will really affect the price - at a guess $150-200 for a basic pair.
If you do want to go down the authentic route, you could look for the
Underground brand in US stores, they're not as expensive as George Cox but still an English brand with a bit of heritage, and should be more widely available in the States. Their classic model is called Wulfrun.