If you've got access to Ancestry.com, and need German city directories, here's an easy way to generate a list of what's available:
Go to the Ancestry.com's "Search" page:
In the right-hand navigation bar, click "Card Catalog."
Under "Keywords", type in "German directories" without the quotation marks.
Voila! There's your list!
BTW, if you don't have a subscription to Ancestry.com, ask your local library if they provide access to Ancestry.com or HeritageQuest.com for their patrons. You may also want to ask them if they've got a subscription to Footnote.com-- and if not, why not?