Hi- This may not be helpful, but when I did the IM there a few years ago, I could not find a bike shop right in Frankfurt that serviced/sold tri/racing bikes/accessories, but did find a shop I could take the bus to. Look in the surrounding towns...i needed brake pads for my carbon rims, and was able to find a shop in a nearby town, but absolutely nothing in Frankfurt, just commuter bikes and accessories. Good luck!

There's a shop listed as official bike rental partner for IM Frankfurt, maybe they can help.

http://m.eu.ironman.com/triathlon/events/emea/ironman/frankfurt/athletes/bikerental.aspx