The first thing I did on this site was work through all the megathreads I was interested in (guns and then found SN and became infatuated).
Click around the suicide discussion part and there are suicide resource threads. Read it all or skip the pages. Should only take a few hours. You can space it out over days if preferred.
Before searching for purchasing SN (sodium nitrite) definitely understand the pros and cons of this method against others. It has a 7/10 peacefulness rating in the PPH (peaceful pill handbook), which is a book produced by Exit International. A great organization that does proper research on methods. But from what I've come to understand, I think 6/10 is closer to reality. And so Benzos are necessary in combination. All this stuff takes a long time to order and obtain.
Take a deep breath.
Anti-emetics are drugs that prevent nauesa and vomiting and are also suggested for the SN method.
There is a guide called "Stans Sn guide" that you should be able to search for.
N (nembutal) is the gold standard 10/10 peacefulness, better than SN by a sizeable amount, but only has 4/10 availability rating). But most people work their way up to it as it costs a lot and is risky to buy, and has only one trustworthy source.