I also hate to spend alot of money on kids costumes because they will never wear them again. There are a few different options, going to a thrift store and making up your own costumes is always a fun idea. Also dollar stores are starting to bring out tons of great halloween masks and accessories.
Every website will offer you discounted prize costumes.
You could look at your local Walmart, Target or Kmart. If you're unable to find anything there, you should be able to find something on a website such as www.costumeexpress.com
i'd say the easiest and cheapest place would be on ebay or amazon...
wikapidia
You can make your own costumes cheaply. For example check out carboncostume.com.
NYC Party costumes sells online and ships daily to Australia
Do not buy costumes from any site other than the YoVille site.
Try partycity.com.They have a bunch of costumes there, but maybe not the one you want.
Wordpress has the cheapest method of setting up a website. What is really interesting about Wordpress and why it is the cheapest is because it is free you don't have to pay for it.
Here is a website that offers kids, teens, and adults X-men costumes: http://www.costumecraze.com/Superhero-Costumes-Xmen.html. This website offers X-men costumes exclusively: http://xmencostumes.net/.
Astronaut Halloween costumes are readily available online. For your convenience, I have provided a link to the website: www.costumecraze.com/Traditional-Costumes-Astronaut-Costumes
There are many places online that offer fairy costumes for children. On eof the best places to find these costumes is the website spirithalloweenstore.com.
Costumesupercenter.com has angry bird costumes already on there website. The price for the costume on there site is 35.97 before shipping and handling.