Yes, I've seen those too. I have a relative who has had one at his house for years and he thinks he just bought it at a local Fred Meyer or Home Depot. If you go to Amazon and type in "plastic chain and stanchion" you'll get hits for stuff like this. If you're looking to bury the posts rather than have them free standing, maybe you can just get the tops from the link below and cut lengths of PVC pipe that you can get cheaply at a big box store.

Here is a link that might be useful: Plastic chain

Kudzu9, thank you very much for the reply! I did look at that link you sent, and I think that solution will work nicely. And not too pricey because they're not metal! I had also thought about checking equestrian websites because I have seen dressage arenas with a similar border. Found this for anyone else interested: http://www.crowdcontrolstore.com/plasticin-groundpost.aspx