Ving, Try a 'Bearing Supply' store. They have heaps of 'O' ring seals.
& yes, you will need the 'O' ring (if a 3 screw version, the tube pivots at the 'O' ring), if you want to collimate the finder. L.
All Bearing shops sell 'O' rings in every conceivable size and thickness (Even round ones LOL). Just take your Finder with you and they will fit the perfect one.
They are usually about 40 - 80cents for that size. Buy some spares while you are there.