There´s a lot of music´shops in the main street.

Sorry, i can´t tell you more of this

Underdog Recordstore:

http://www.underdogrecordstore.de/

Mostly hardcore, punk records. Some indie stuff too.

Normal Records:

Vogteistr. 18 (don’t know the webpage www.normal-records.de seems wrong)

Great indie record store.

Kompakt record store:

http://www.kompakt-net.com/

Electronic music

Grooveattack:

http://www.grooveattack.com/

Hiphop and broken beats.

Hope this helps.

suggestions from someone I know, a writer w/a music magazine who lives in Koln:

Kompakt, they are world famous for minimal techno and also sell other electronic music

Groove Attack for Funk, HipHop, Latin, Jazz, House Music

A-Musik for avantgarde, electronica, folk, obscure stuff