Well if you want the standard White colour 35 then try DIY shops, they're likely to to still have stocks left. Almost all T12s available at the moment will be Halos so they shouldn't be to hard to come by. There's online shops too like the one Bulb Freak's just posted a link to, the trouble is with VAT and postage it's often not really worth it. If you're after some of the Deluxe halophosphate colours like Kolor-rite, Northlight and Deluxe Natural you could try The Lamp Company:
http://www.lampco.co.uk/results.asp?section=Lamps&type=Fluorescent%2FStripThere's also Ebay, there's normally plenty of colour 35 T8s on there!