I'm just saying it is a stupid idea. Why would you take SN and then "the antidote." You aren't going to find a "source" because the entirety of methylene blue usefulness in a medical setting is to administer it IV to reverse met-hgb. Your logic escapes me rambling on about worrying SI will kick in and simultaneously wanting to buy methylene blue.
here is the absolute best (factual) answer you will get on the subject: Don't take SN unless you want to CTB. In an acute care setting, prompt administration of MB can reverse the met-hgb in the best case scenario when administered promptly. The chances of oral methylene blue break down to extremely likely it will do nothing, very remotely it will prolong the dying process, and almost absolutely won't stop the process.
if you want it, buy it