Best thing to do is to take the power supply to an electronic store and see if they find you power supply with matching specs. 

In my area, the SBG8300 is compatible up to 800 Mbps speed plan with Xfinity.  Might be slightly different in your area, you can check  at http://mydeviceinfo.xfinity.com/

I usually just source them from amazon. they just need matching voltage and polarity. You can have higher amperage but not lower. 

My area supports the sbg8300 up to gigabit. They say “its not compatible” with certain plans because its only got a gigabit port but you can use it on the up to 1.2gig plan. You just will only get gigabit speeds. 

Thank you for the feedback--very helpful.