When in doubt, ask the director/cosplay staff of the convention where you plan to enter.
As a judge, I will say this. It doesn't matter where you get the pieces of your costume. We judges are cosplayers too, we KNOW you don't make wigs from scratch hair by hair (generally), nor cobble your shoes from the soles up with raw hardware. We're aware of this. It's not like we're going to penalize someone for not learning to be a cobbler or a wig ventilator just for the sake of a costume. If you didn't actually do any work whatsoever on a piece of your costume - no styling in a wig at all, not even trimming or spritzing it, no alterations or any work done on your shoes or prop at all - we just don't look at it. It doesn't count as a part of the costume being judges. We judge what you DID do work on. As long as the rest of the costume has enough work put into it to qualify for the rules of that masquerade (which vary from con to con, which is why I say to ask the director/cosplay staff first), then we don't look at your shoes, wig, or prop the same way we don't look at your store-bought bra and underwear. They contribute to the overall look of the costume and that IS important, but workmanship judges don't judge things that haven't had any workmanship done on them. We place a convenient mental blinder over it and don't take it into account.
Whatever you do, the overall costume needs to conform to the rules, so pay attention that first. Some cons allow you to even get only one part of the costume judged, like a prop alone or the fabric part of the costume alone. Some don't. Some are more strict than others when it comes to quantity of the costume handmade - some say 50%, some 75%, some more, some less. And only the director or cosplay staff/judges at THAT CON can tell you whether your costume qualifies for THEIR rules. If they say it's okay, then it's okay. Go in, be HONEST, don't try to snow the judges and pretend that you handmade a prop that's obviously store-bought. Like I say, we're cosplayers too. We KNOW.
And novices do get a lot more slack for relying on pre-styled wigs and completely un-altered shoes and pants and things.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com