Usually you can buy 300ft (100 meters) of electrical wire at many many online-etailers (no m0dd1ng-shops) for about 15-20 Euro (Dollars).
A little bit of searching should bring results, no matter where you live

American size:
AWG18 for everything except fans, front-panel-things, USB
AWG26 for everything which is not covered by AWG18

European equivalents (if you want to search in Europe):
AWG18 = 0,75mm˛ wire
AWG26 = 0,14mm˛ wire