if it's a prop made out of cardboard or wood, I don't think any of the security are going to think it's real and confiscate it. it will feel much lighter than a real knife, and unless you're a master effects painter it won't look the same (probably just fine for photos though
) so I wouldn't worry about it being mistaken as real, just read up on the specific cons rules about props. there are some interesting tuts on the link that Ditto provided, I'd check that out.
If it were me, I'd totally make a prop butterfly knife for the Spy. being able to do work a butterfly open and close is really easy to learn (I think I did it in 1-2 afternoons, on both hands, while reading or watching tv). and it'll look very cool with your costume.