U.S. Rotax Retailers

Who are the U.S. retailers for Rotax?  I conducted some searches, but did not find much.

I am building a Sling TSi and will either go with a 915is or 916is.  Sling offers Rotax engines with the kits, but I'm uncertain if there is an advantage to buying directly from Sling, or if I should consider other sources. 

I like to do my due diligence and want to understand my options.

Thank so much!