I've heard that belladonna can be used to treat nausea?

Not recommended. Belladonna is commercially available over-the-counter (otc) for short-term use as "earache tablets" and as a "leg cramp" preparation. Belladonna can not be recommended otc as an anti-emetic, nor without physician oversight or prescription would safe dosing be possible. If you have nausea that persists, the list of possible diagnoses is very long. Please get check rather than self-medicating.
Homeopathic Belladon. Belladonna is one of over 800 homeopathic remedies that can be used for nausea. This doesn't mean all nausea would be helped by belladonna, though. To find the best remedy for your nausea, a homeopath would explore all the characteristics of your particular case. Homeopathic remedies also are prepared to act in concert with your own restorative system; this process is safe but powerful.