Scout from TF2 is a very easy cosplay. All you need is a red/blue shirt (That's up to you), a bat bag, a black cap (Or any of the other hats you can wear in game), a headset with a mic, dogtags, dark green pants, high socks tucked into the pants, black shoes (Sneakers or cleats), and a baseball bat.
Reason I say he's easy is because a lot of the props/costume pieces require little modification and can easily be bought new or at a second hand store like Goodwill. The only limit is body type. If you're on the bigger side, I wouldn't try this.
Another TF2 character who's easy is the Spy. You just need a dark blue or red suit (Again, up to you), a balaclava (You can find those on eBay), black gloves and dress shoes, a watch, and a toy revolver.
As for his knife, most cons probably won't allow a real one. Thankfully the Spycicle just came out from the last update. The fun part is the disguise masks. You don't have to stop at the other team members, you can do masks of other things like internet memes (Trollface, Rage Guy), other video games (Portal Cake, Mario), anime characters (Inuyasha, Naruto, etc) the possibilities are endless. Get creative.
Now for the pics.