There's a few considerations & options, depending on how much money you want spend. Pitch is important, as Indian instruments are always significantly sharp of concert - not a problem for solo ballad singing, but if there's an outside chance of you playing with other musicians it will be!

Shruti Boxes by their nature have all twelve notes of the standard Western scale, which you can choose from for your drones, changing as you sing as many Shruti Boxers do, treating it as a slow-moving free-reed-ophone rather than just droning per se. So you can play in any key - however so sharp, as I say.

Ebay? Hmmmm. The last free-reed instrument we bought from India was a ridiculously expensive ultra-portable harmonium which looked like it had been made by monkeys complete with hazardous razor sharp metal decorations and SO LOUD it took a lot of modification to make it remotely useable. Beautiful tone though! We used a few times live, but owing to pitch issues it was soon retired for Studio Use Only. See my wife playing it HERE.

So beware! Check out British suppliers which will lessen the potential for heachaches. Jas-Musicals is a one place to look, and there are other UK suppliers who do nice SB's tuned to concert pitch with softer reeds than you tend find in Indian models, for more Westernised senstivities & singing styles. See HERE for one option.

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For my own ends as a Shruti Ballad singer, I use an electronic shruti box (yes, off EBAY!!! An impulse buy!!! See HERE). This ugly-but-durable nasty piece of plastic gives 48 tonic / unison / 4th / 5th drone options and is fully tunable with a handy volume control and built in metronome (which I've never used). Seen years in my keeping (touching wood) it has never failed me & is used in every gig we do. For more complex / organic modal & Shruti drone patterns, I use an old style Korg Kaossilator, which is a gift for balladry as far I'm concerned. You can hear both my electro shruti box & the Kaossilator being used for balladry HERE.

Lastly, there is an ONLINE Shruti Box which you can play live on your laptop HERE. Great fun!

Wishing you happy wandering in the land of Freedronia...