Boot Glides  

There's a seller on eBay...
There's a seller on eBay that has been there for years ... I can't remember the name but it might be DePury. They also sell bases, etc and seem to have a lot of satisfied customers.
Am I wrong, doesn't HM sell these?

The glides from Graham...
The glides from Graham Mancha are genuine Herman Miller spec and made of rubber, I think the ones on ebay are made of plastic and are glued together. I guess being rubber, over time with heat and use they perish? Having said that I have had a set for years in daily use with no issues.

Hume?
I've never bought from them, so maybe others can comment.
http://www.humemodern.com/parts/parts.html

depury
I purchased boot glides from him. No complaints.