There are some of us who may, if they're lucky enough to find it, the Dealer/vender's specialty is that particular character they are cos playing, or just so happens to have general items /accessories you could use/add onto your costume, you could get it right there at the 'Kon itself. No real guarantee that you will find that perfect or matching skirt/ handbag, or that just the right sized Bushido blade for your costume (funny... to see a dealer selling only a whole assortment of swords, axes, and blades like a medieval blacksmith right out of a Mel Brook's movie, in the dealer's room. "Get your swords, get your axes, katanas... Hurry! Hurry! While supplies last! Hurry! Hurry!"), but it might be good knowing if you need a back up prop, you maybe able to pick one out without leaving the premises to travel all the way to Ala Moana, Don Quixote's, or even that costume shop clear across to the other side of town.
I've witnessed several cosers improvising and doing something like that just to get their costume completed in some way.
For of course it likelihood will not be that convenient. If it was, where's the challenge in that?
Huh... or maybe even have an "on site seamstress" who can help sew up that little rip or unraveling hem in your costume. for a small fee, or at no charge. That would be nice. If the owners would approve having one or two on site to do that as volunteers, great. No pay, but the PR would be tremendous.
I favor that kind of convenience. My "Mystery Kaijun/kaijin" is still under experimental design testing, meaning, the material I will be using still needs to be determined for strength. And since I'm not a total whiz at sewing, I might need assistance and a back-up plan of sorts just in case my costume decides to go south on me by coming apart at some seam.
Hopefully, not at some seam on me that would be really totally embarrassing...
