There are a couple of options depending on whether you want free or paid for training.
Trailhead is free and self paced training with specific trails for Salesforce Administrators at Beginner, Intermediate and Advanced levels. These are not so directly tied to the certifications as the training courses but cover a lot of ground including the 3 releases each year.
Salesforce University has a variety of packages available depending on the experience level of the student which are directly tied into the Administrator certification path. These are all paid for training. The most suitable for beginning administrators is the ADM201C or ADM202 if you are using Professional edition.
Trailhead is great preparation even if you are booked to do an instructor led course as it includes plenty of hands on exercises and can more easily be fitted in with work as it is broken up into units many of which only need around 15-45 minutes time to complete.