Hi Roger.
No one in Bali I met like to do this kind of business, but in my opinion go ahead for your business, that's great.
Answering your request, I can't tell you where in Bali that you can buy a mini guitars (I've seen the cheap one) and I'm not sure which one or what kind.
In Indonesia there are a few mini guitars, the 'KECAPI' (read: kechapie) used for KERONCONG (read; c'rwonchong) instrument which I doubt you'll see in Bali, the nearest place to find it would be Surabaya or Jogyakarta. Every islands in Indonesia has their own instruments, like Nortern part of Sumatera, the instrument known as 'olele', it is also a miniature guitar, Iam not sure what is your idea about miniature (size). If you ever been to Hawaii (USA), in the big island there is a manufacturing of this kind (Hawaiian guitar) also you'll find in Indonesia as KAPOK brand which assembled in Brastagi High Land (Sumatera) and some in Java.
For your best hunting in Bali, I guess try go to music stores.
Kind Regards,
Eddy S.