I’ve a mixture of the o-ring mounts and out-front mounts and swap between them. I wouldn’t re-use the o-rings after fitting once, just think they may have stretched to fit one bar then not quite fit the next etc. etc.
e-bay have loads, would hazard a guess that you pick a supplier and when it asks for quantity, you add as many as you need.
Oh, have to add..
When I use mine on the CX’er (which has the out-front mount on it) if I’m riding rough (proper rough) terrain I wrap a thick elastic band around the unit, then around the mount to secure. I counter that by saying I’ve never had the Edge come off in any ride I’ve done, but when on the CX’er I kinda think it’s more secure with a band around it. Also if you wrap the band towards the top of the unit it just clears the screen so you can still swipe.