I bet you can get them at J&P cycles they have everything.
http://www.jpcycles.com/
I have used many parts from them and they also have excellent customer service!
I have speedmaster risers and america bars and like the riding position. I have heard from others that the Hardley risers have a slightly larger bolt and our trees need to be drill slightly larger. Not having used risers from another maker I couldn't tell you for sure.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!