I would use purl.org yes. That's weird that they're not answering. They answered pretty quickly when I tried myself. There's also https://w3id.org/ if you're looking for stable and persistent identifiers.
The best however could also simply be to publish the vocabulary yourself. One of the issues with using URIs for the vocabulary of your data is that there's so many to choose from already: http://vocab.cc/
Instead of creating yet another vocabulary make sure the concepts you're trying to identify don't already exist. Also http://lov.okfn.org/dataset/lov/ is a good place to search for existing vocabularies.
If you feel you need to publish new URIs, then try to go with something persistent unless it's so specific for your use-case, then you should perhaps use your company's domain name.