There's no direct installer available for Skype for Business, as it is part of Office Pro Plus, but if you have enough with the basic version (i.e. you're missing a couple of enterprise grade features), you can get Skype for Business by first installing Lync Basic, then install the required updates to turn Lync into Skype for Business.

1. Microsoft Lync Basic 2013
2. Install these two patches:
   • April 14, 2015 update for Lync 2013 (Skype for Business) (KB2889923)
   • April 14, 2015 update for Lync 2013 (Skype for Business) (KB2889853)