008 Buna-N O-Ring, 70A Durometer, Black, 3/16" ID, 5/16" OD, 1/16" Width (Pack of 100): O Ring Seals: Amazon.com: Industrial & Scientific
BS112-12.37mm x 2.62mm (1/2" / 12mm) Imperial Size Black Nitrile Rubber"O Ring" / Washers x 500, to Suit John Guest Fittings Amongst Other Things : Amazon.co.uk: DIY & Tools