I don't think that Nitromethane (what we use) and Nitrous oxide (laughing gas) are the same thing.
CH3NO2 -nitromethane- is a liquid and NO2 -nitrous oxide - is a gas
I seriously doubt that a medical supply company is also going to distribute a racing fuel.
You'll be far more likely to find nitromethane at a shop that works on/builds high end racing engines or a race track or road course. I'm sure there's one on the African continent somewhere. South Africa should have one for sure.