You need a dopamine blocker with SN
Zofran is a serotonin blocker. :)
I wish you could use Zofran as I've already got that and it seems to have less side effects than dopamine blockers. Also, as you point out, much easier to get in the US.
From SN Megathread:
Antiemetics: Strong enough antiemetics for this method are not OTC, so you'll need to see your way around this. You're looking for Metoclopramide or any of it's commercial names, like Reglan/Primperan. This antiemetic needs to be a Dopamine blocker for it to work.
Antiemetics aren't 100% necessary for this method, however; just like with N, you wont accomplish anything if you end up puking it all out, which is likely. This will also most likely be the bulk of the money you'll use during this method.
Acid Regulators: Another thing recommended in the PPH is raising your stomach's pH to make the SN more effective. This can be accomplished with drugs like Tagamet (The one recommended on the PPH, 800mg, which is to my knowledge OTC) or even bicarbonate, though not as effective.