Try meclizine (Bonine/Dramamine Less Drowsy) or cyclizine (Bonine for Kids), they're both OTC and meclizine is a fantastic anti-emetic since it works on 5-HT3 receptors like Zofran, has mild-moderate anticholinergic activity, and may also affect the medullary chemoreceptor trigger zone, all of which work together to reduce nausea.
I haven't been able to find cyclizine proabably due to its stigma stigma and the fact it became OTC in the US only 3-4 years ago in 2009.
If you can get an Rx, try Hydroxyzine at 20-30mg, which is my hands-down favorite anti-histamine due to its lack of significant anticholinergic effects (which can really detract from the nuances of an opioid high), 5-HT2C antagonist effects which reduce anxiety, and for its long duration. Promethazine has a similar duration, but is far too sedating at moderate doses for my tastes and acts as an antipsychotic at high doses.