AFAIK, there isn't a really good resource in English. Spalink's Die Deutschen Hufeisenstempel
is the gold standard reference, but it's in German.
An excellent online resource is at http://www.hufeisenstempel.de/
. I believe the proprietor of the site is also responsible for this
On the website, if you click on the Index, you are presented with a list of cities. The page of each city includes links to examples of horseshoe postmarks of that city at various periods of time. There are dozens, if not hundreds, of examples.