Here you go...http://www.shop4fasteners.co.uk/acatalog/Security_Machine_Screws_Pin_Torx_Button_Head_Resistorx_BO.asp
The vendor is in Sheffield.
Trouble is, I don't see a way to buy smaller quantities, but you could split the cost with friends. If it weren't for the overseas postage costs eating up any savings, I'd probably buy a box and offer them at cost under "Bike Bits" in the Private Classified board here.
Yes, a person would need to chuck them in a lathe or an electric drill and turn-down the heads a bit using a file or abrasive, then a nice polish of what remained, as the finish looks a bit rough.
Here in the US, we have a purveyor of cheap-good tools (tools that are cheap and cheaply made, but suitable for purpose given the cost...sometimes surprisingly so!) who now regularly sells kits of bits to remove these security Torx bolts. That means they're only secure against those who haven't (yet) spent USD$3 for the kit.
Nothing is absolutely secure, but a Torx security bolt would force a determined thief to carry more tools or come back a second time, and that might surely help. And, compared to other solutions, pretty cheap.