The 6mm X 3mm 90 durometer O-rings are available from [link=http://www.rocketseals.com/]Rocket Seals[/link] as P/N 6mmX3mmB9 at $0.22 each, minimum quantity is 100. It is not a stock item so you have to call and there is a 7-10 day lead time.

They are also available from [link=http://www.motionindustries.com/motion3/jsp/mii/index.html]Motion Industries[/link] for $0.64 each in quantities of 100. Just order by description.

You can call most O-Ring suppliers and tell them what you need and they will get it for you.