Prochlorperazine (Compazine) is heavy duty stuff; I wouldn't try to find any or worry about its efficacy. I would just use OTC Dramamine. Some studies suggest it's about as effective as Ondansetron (Zofran) and it's much cheaper.

I am not familiar with Promethazine (Phenergan), but some quick research shows it is an antihistamine. Not that antihistamines are automatically safe, but generally I would advise trying an antihistamine antiemetic before using something like Compazine.

Also, there's nothing wrong with trying more natural methods to reduce nausea, such as ginger.

Are you on any sort of anti-anxiety medication?