One of the most common uses for Sodium Nitrite that I know of is meat curing. It helps to know about at least 1 of its uses (outside of CTBing of course) if in case you get asked any questions. Sadly I don't know much about trying to get your hands on SN because I've never tried myself, but I know it can be quite difficult to get in the UK. I'm not so sure about outside, but I have read some places where you can buy SN require you to give them some form of ID and be over 18. SN should rarely get stuck in customs, but it has happened before which is why knowing its uses can be useful in the rare case it somehow gets stuck/seized in customs.
It's not very painful but rather more uncomfortable. Common symptoms while undergoing SN overdose is increased heart rate, nausea and bluish lips. If everything goes smoothly it takes about 30 mins to fall unconscious. Not correctly following the procedure can induce vomiting, which is why some people make 1 or even 2 backup glasses just in case. Last failed attempt I read was because that person did not make a backup glass. So if you ever do decide to go with this method, I highly urge you to make at least 1 backup glass if in case you vomit the first one and still don't want to back out of CTBing.