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I've seen many good Scrummaster's become excellent agile coaches so you are definitely heading in the right direction. For me, the difference between a Scrummaster and an agile coach is the breath of influence a coach has. A Scrummaster typically works with ONE team- mentoring, coaching, and facilitating the Scrum process. An agile coach typically works with MANY teams plus senior managers/executives associated with those teams. Coaches enable agile capabilities from top to bottom within an organization.
There are many agile coaching training and certification classes available in the market. A quick google search will find classes (and tracks of classes) from Scrum Alliance and ICAgile that may be what you are looking for. Regardless of which one(s) you choose, make sure the focus is on how to work effectively across teams and with senior management. Good classes focus on strengthening your overall facilitation skills as well.
One last note is to not expect to become an expert coach overnight. It takes much more than a class to become a great agile coach. Training may get you started but it will take hard work and experience in the trenches to gain the breadth of knowledge necessary to become one of the best.
Good luck in your agile coaching journey!