High-quality Devanāgarī fonts suitable for professional typesetting are still hard to come by. One foundry producing fonts to something like the required standard is Pune-based Modular Infotech. They offer true bold faces and true italics. Refer to their specimen (published 2004, but apparently still current):
Since I haven’t used any of Modular Infotech’s fonts at the time of writing – they don’t come cheap – this is not yet a recommendation. Meanwhile, it’s possible to do some limited testing at fonts.com by clicking ‘TRY IT’ and typing a Unicode Devanāgarī string.