You'd probably be better off reading the full guide by Stan or the Peaceful Pill handbook. It will give you more information and probably make you feel better than taking it second hand from us. But anyway...
It's easy to get in the EU but depending on the exact country it can be easiest to order from abroad rather than within the country because it is quite restricted. People here have sources, such as IC. But I do know people in Poland and some other countries got it directly.
It's very cheap as a chemical, but it depends. If you're lucky enough to get it from a chemical supplier, it is dirt cheap. If you get it from a middleman source like IC then it costs way more, but not more than 100 euros or so.
You don't have a whole cup. Read the guide. 50ml per 25g of SN. Most people take 25g but if you're very light or heavy this is adjusted.
How long to get knocked out? Rough estimate, 10 mins or so. Could be longer, or less. Hard to predict. Is it painful? Again, hard to say exactly. It has some uncomfortable effects like heart racing, headache, nausea, vertigo, sore throat, maybe some stomach discomfort. Potentially uncomfortable at the least, maybe some pain, nobody can guarantee no pain but the accounts seem to be it's mainly discomfort. Benzos are recommended to help reduce the impact of these.
