I ust got 2 cans from worldmagicshop.com (£11 quid ish each) and its called "Testors All Purpose DULLCOTE - 1260 Flat Clear Lacquer"
I've had several roughing sprays / fluids, and this is BY FAR the best I've used - it sprays nice and evenly, doesnt "blob" when sprayed and the only reason that you can tell I've used it is because I tested the spray on some old tiger cards (So if you intend to use the spray on black faced cards, bare in mind that it WILL make the face look slightly matt when directly compared to another black faced card). White cards, red or blue bikes for example, are fine though.
If you like I'll send a quick piccy of the blck card, so you can see what I mean?
Dan
http://mfpilot.com/images/Testors1260DullcoatSpray.jpg