I was introduced to this instrument on a road trip (some time ago). Vishwa Mohan Bhatt was playing on a CD (A meeting by the River) with Ry Cooder and asked what type of sitar Bhatt was using since it sounded (sort of) different.
Needless to say, I was intrigued by the response ("That's not a sitar - it's a Mohan Vina").
Some research revealed that it was indeed a converted/customized /modified guitar played with a slide. I have been trying to find out whether or not these are commercially available or if anyone has the specs on HOW TO make this instrument. I am somewhat handy at making guitars so I may try to make one. Maybe someone knows Vishwa Mohan Bhatt and he can reveal his secret??
Thought I should let you know of some other players of Hindustani Slide Guitar. Other than Vishwa Mohan Bhatt there is also Debashish Bhattacharya (who studied slide guitar from the exceptional Pandit Brij Bhusan Kabra) and Barun Pal (among others). The original idea for playing Hindustani music on a modified western guitar is generally attributed to Brij Bhusan Kabra, not to Vishwa Mohan Bhatt (he just decided to re-name the instrument Mohan Vina [after himself] after making some modifications). If you want to hear more of this instrument (and in a traditional Hindustani setting, which Meeting by the River is not) I would highly recommend the two recordings of Debashish Bhattacharya put out by India Archive Music. These come with very good liner notes including information on the history of the instrument (the second release, "Hindustani Slide" - Raga Bhimpalasi, has more extensive notes including some minor specs on modifications, but not enough to tell you how to build one).
In terms of actually purchasing or building a Hindustani slide guitar, I'm not sure what to tell you. My best recommendation would be to try to contact Debashish Bhattacharya directly through Lyle Wychovsky at India Archive Music, 2124 Braodway, Suite 343, New York, NY 10023. My guess is that Debashish would be more approachable than Vishwa Mohan Bhatt, but I don't really know.
Good luck! Hindustani slide guitar is really a great sound and I hope you are able to find or build one.