Halloween costumes can be the cause of such stress! They're cheaply constructed, generic, and expensive.
Some thrift stores set up Halloween sections during this time of year. If you live in Los Angeles, at the moment there is a large section in the Goodwill at the corner of Santa Monica Blvd and 6th St in Santa Monica. I'm sure other thrift stores have a similar set up. They may not have the hottest superhero your child most desperately needs, but they're likely to have more unique options (no one wants to show up wearing the same thing as someone else), and still allow your kids the fun of roaming the aisles to pick out his or her own costume. I think it's also a good chance to instill that Halloween really is about having fun, getting dressed up, and stuffing yourself with candy. And just as much fun can be had in a used Gumbi costume as in a brand new Iron Man costume.
I've also heard of parents organizing costumes swaps. Both are really great ways to get kids fun new costumes on a budget, and while helping to minimize clothing manufacturing and landfill use. If you're in New York, go to this one that my friend is organizing on October 24th !
Another idea is to convert an old costume into something new. With new accessories a fairy can be adjusted to a princess, remove the padding and get a new nose and an elephant becomes a seal, dye a good witch dress and it becomes a wicked witch.
You can also make your own costume. All of my favorite costumes growing up were made by my mom. They were nothing too elaborate, but it ensured that no one else would have the same costume, and I got to help her out and suggest changes here and there, so I got just what I wanted. We also stored costumes away, so I often wore what my brother had worn a couple years prior. They also had a great homemade touch--accents and accessories were actual tactile objects, not just screen printed onto chintzy fabric. Scrap fabric, string, glue, tape, paint, and carboard boxes can take you a very long way. It's so much fun to get creative with kids. Making costumes with them is a great way to stretch their imagination and encourage them to find ways to translate that imagined idea into a functioning frock.