But are you aware of two free ones that are quite useful?
The first is well known, and that is Internet Archive (www.archive.org). Just enter "city directory" with the quotes in the search box and depress the "Go" button. At the time of the publication of this post there are 1,696 results. You obviously would wish to refine the search by entering your city of interest, but it tells you that there are a lot of city directories available to you.
The second site, (Don's List) has a far less number of directories available, approximately 10% of the Internet Archive collection, and also includes some international directories. You can find the directory links at http://www.donslist.net/PGHLookups/Dir1Win.shtml. The international links are on the right side of the page about half way down.
I have not done a comparison to see the overlap between the two sites, but either of them are extremely useful for finding out where your ancestors lived. And in many cases, their occupation and given name of spouse is provided, depending on the directory.
City directories are an under-appreciated form of research. Lots can be learned and if your ancestor owned a business, even more can be ascertained.
Give it a try!