You'll want to do a little bit of reading about them to decide what you want. There are a couple of concentrator vendors who are LE vendors. (Forgive me if I'm forgetting someone.) Look in the "Sale Rack" section of "The Mall" section for posts by Justin L of Oxygen Plus and Kimberly (kbinkster) of Pyronamix. They'll have links to websites in their signatures of their posts.
There are vendors who sell concentrators on eBay. People have had different experiences with getting concentrators on eBay -- some good, some not. Before you go that route, check out the vendor carefully. You might want to post a question here. You should check out the vendor's feedback using
www.toolhaus.org, which is a site that lets you see all the negatives and neutrals that a vendor has gotten including feedback that they've talked people into withdrawing. (There was a thread about this recently, and someone mentioned the toolhaus site.)
Here's a link to a thread about problems with buying a concentrator on eBay -- but there are people who have bought on eBay and been happy, too.
eBay concentrator thread
Looking for oxygen concentrators.
Oxygen Concentrator Sales
