I went to Diehl & Diehl on the rec of a number of threads in this forum. Really nice store. Picked up a few things on sale (end of the sale season here).
Hahner an der Oper (aka Willi Hahner) was also pretty nice http://www.yelp.com/biz/hahner-an-der-oper-j%C3%BCrgen-hahner-frankfurt-am-main
These places had nice accessories like Magli umbrellas, fancy socks. Diehl & Diehl had an incredible shoe selection.
As you walk along Zeil, there are a lot of stores as well,...