The simplest; jeans & leather jackets for the punk look; then raid Savers/Goodwill for loud colorful jackets, mostly you will have to shop in the woman's section, for the glam rockers. Same stores will also spill frilly shirts w/ puffy sleeves and keep an eye out for T-Shirts especially one's shrilling for musical groups or simply having the logo of a bar or tavern. Also scope out above mentioned stores for mustard colored tailcoats for the 50's rockabilly, also tailcoats in off white with brown piping - - - a pair of eye glass frames and you have Buddy Holly. Wigs truly make the look; nearing Halloween you should be able to score them for $10 to $15 each; long hair, strange colors, and always the all purpose Afro. Also through Oriental Trading (go online) you can get inflatable guitars cheap, lay in a stock of a dozen, have them around. PS an obvious one is to invite your guests to come as their favorite Dead Rock Star.
Wanna get creative; create a Juke-Box - - - a walking talking Juke-Box, one person I knew wore a battery and had lights & a turntable visible behind clear plastic front - - - he won a $50 prize. Other 'gimmick' costumes would be a Guitar "c'mon baby, play me," Have a rocking good time.