The Online First articles are reachable from the Volumes and Issues page for the journal. The direct link to all current Online First articles for any journal that has them can be found on a page like this:
http://link.springer.com/journal/{JournalID}/onlineFirst
For example,
http://link.springer.com/journal/12603/onlineFirst
It is also possible to use the RSS feed button on the journal’s home page to keep up on the latest articles, which would include Online First articles.

Example: