It depends.
If you will like to work for Ketron, then contact the main site.
If you will like to be an independent provider, then you will need to buy an Audya ( Audya5 is my suggestion) and start to create new styles and sounds.
I think that there is opportunities for programmers to create new styles and sounds for Audya and sell it in the internet.
There are no many sites where you can purchase then.
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Machetero