For those of you who don't want to shell out $10 for Garry's Mod: If you have any game in the Orange Box (Half Life, Portal, Team Fortress 2), you can download the Source SDK for free. It has a model viewer which allows you to rotate, zoom, and pan said model in 3D space with game-accurate colors, AND includes all of the in-game animations (plus a few interesting Easter eggs). Plus it updates as the game updates, including unlockable weapons and hats.
There's also two threads on the chan, http://tf2chan.net/workshop/res/22.html and http://tf2chan.net/offtopic/res/1549.html, and the Team Fortress Wiki http://wiki.teamfortress.com/wiki/Main_Page
2) Class patches:
Iron-on transfers and stencil+paint are alright in a pinch, but I'd definitely suggest at least doing the yellow parts in thick cloth so you have a distinctive shape. (Or, if you're feeling REALLY ambitious, embroider one yourself! Or see if you can find someone with a programmable embroidery machine.)
Here's where you get to be really creative. Play with various materials (so far, I've found foam core to handle much better than cardboard) and look at everything through the eyes of "could I make a prop out of this"? Team Fortress 2 is awesome in that you could pick up a lot of stuff right out of the hardware store, though obviously you'd have to check with your con to see if it'd be allowable.
If you don't mind shelling out a few more bucks, take a walk through a toy store. At least in the States, toy guns are required to obviously look as such. BB guns would probably not be allowed at most guns, but Nerf ones (i.e., firing foam bullets) might, though even those would most likely be modded not to fire.
The next level up from this would be to buy from the same places the movie-makers do for props. These tend to be "you WILL get shot holding one of these in public" realistic, so it's definitely advisable that you mod it yourself after you buy one. Check out www.inertproducts.com for one such vendor, mention a friend of yours asked for a brochure at I/ITSEC 2010 and that you're looking for props for costuming. (They usually don't sell to the general public, but they are willing to make exceptions.)
For small props (say, no bigger than an average person's shoe) that you want to look EXACTLY like the actual thing, Google "rapid prototyping". These guys can use a 3D printer to make replicas. But these are hideously expensive?: one replica runs around a couple hundred bucks.