Worldcon - World Science Fiction and Fantasy Convention. The first fandom convention (over 60 years running and still going strong) and where all the big names go.

Costume-con - A convention just for costumers! There's multiple masquerades, panels on all sorts of different techniques, and pretties for you to ogle in the dealer's room. A costumer's paradise! Check out the past Masquerade winners in the "Timeline" section!

Cool Neon - Electroluminescent fibers

 

The Monster Makers - Mask making and prosthetic supplies

 

The Ring Lord - Chain Mail

 

Tandy's Leather - Leather & Leatherworking supplies

 

Kryolan - the best theatrical makeup I've found

Folkwear Patterns - hard-to-find patterns such as kimono, hanboks, and other cultural-specific costumes

Farthingales - supplies and info for making corsets, bustles and hoopskirts

TAP Plastics - for making armor and such

Wildcat Wigs - low-price, high qualityEbay wig seller

Metaldesignz - chainmail supply

Dharma Trading - silks, dyes, batik, fabric paint and everything inbetween.

Feather Place - great to buy feathers in bulk or larger packs, for wingmaking, etc.

ElectriqueBoutique - cheap and unusual shoes

Discount Dance Supply - Great for bodysuits and leotards, etc. Some of the leotards are even dyeable.

Create For Less - discount craft supplies. You usually have a buy in 'bulk,' usually about three packages. But you'll spend less on 3 here than on one in the store.

PMCSupply - all about Precious Metal Clay

FAQS

Armor - Blade Turner Armory

 

Fursuit/mascot making - Fursuit.org

 

Cosplay Closet Tips & Tricks FAQ - info on everything from the right glues to finding sheet metal.