The last time I bought one of those prepackaged costumes, it was over priced (surprise) and fit okay but not great. I would have liked it in a size larger but what are you going to do? I have spent a lot of time searching out stores that have some extended sizes but they end up being incredibly expensive. Why would I spend $50-$70 for something I'm wearing for only a few hours that will invariably end up with some food or drink stain on it? Chances are I won't wear it again another year either. If you are into spending that kind of money, Torrid would happily take your cash. They have various Halloween styles that are $50+ and are sizes 12-28.
What about those who can't afford to spend that or simply don't want to? Make your own costume. You would be surprised at how easy this is and how inexpensive. Unless you want to get into something more intricate with sewing great detail and so on, you can find most pieces you will need at a local thrift shop. They always have various items that can be pieced together to create a unique costume that actually will fit and won't break the bank.
- Pirate/Gypsy/Witch: Look for ruffled blouses, corsets, long flowy skirts, scarves to wrap around your head and old costume jewlery to accent your outfit. If the outfit doesn't look distressed enough, you can use an exacto knife or razor to shread some of the fabric in small areas then run it through the wash. It will come out looking a bit worn and faded.
- Disney like characters: I've seen more adults wanting to dress like Cinderella or Belle lately than little girls and that's great. It is easy to find similar style gowns and the accessories on the cheap.
- Various Animals: These are easy enough depending on what kind you want. A little makeup, headband with ears or horns and you are good to go.
- Medical Staff: Most thrift stores don't carry old scrubs but Walmart actually sells scrubs. If you are a great seamstress or know someone who can sew, get an outfit, cut it up to how you want and boom - costume.
- Cop: Same goes for being a Cop or a Sexy Cop of some kind. Finding the various parts of the outfit is easy enough and fake handcuffs aren't expensive either.
- Various Book or Literature Characters: It is very easy to find things to put together so you can look just like your favorite character. Especially since a lot of book characters channel the fashion of years ago so finding the old plaid or worn clothes is an easy find.
For all the other special additives such as fishnets and tights, I have found that Burlington Coat Factory does sell it in plus sizes. I know Walmart does sell it in plus sizes as well but I haven't tried them personally. I may this Halloween season though. I invite you to be creative this year and find something that you like and want to be. Don't feel limited or intimidated by what is on the rack because it isn't your only option.
Good luck, stay safe and enjoy the sweets!