I have to agree with Rob. Either buy yourself a proper helmet - and I would recommend a full face FIA approved that you can also use on a bike (you will need it for open top cars) or dont buy one and simply pay £10 a pop for renting a good quality helmet on track days. After all, its your head thats being protected. Open face helmets and standard seat belts dont work very well as you will simply slam into your steering wheel/airbag. So unless youre using a harness, I would go for full face.
I wouldnt buy a second hand helmet. If its been dropped once, its no longer valid.
Places to buy them include DT, but also check
http://www.grandpri.comwww.grandpri.com (London, Silverstone) and theyll probably give you a discount.