I would start with suggesting reading the megatread
Exit Bag and Inert Gas Basics —A very, very long post, I know, but I feel like I've fielded a lot of questions about this, so I figured I'd spell out everything I know. Others on the forum should please add to this thread whatever reliable technical information they feel is appropriate for...
sanctioned-suicide.net
But to answer your questions, it's not generally recommended buying helium or "ballon helium" online because they have been known adding oxygen to stop people ctb. Industrial helium is still possible but it's up to you to trust the sellers rating of the gas. Most use nitrogen because of availability now and works in the same way.
You will need approximately 600l of inert gas for guarantee ctb. Compressed that would be 3 liter compressed 200bar
An ordinary oxygen mask is not recommended because it's not designed to keep oxygen out, if you're going with mask it needs to be completely sealed and the most popular methods are Scuba, scba. The most simplest solution are the exit bag with inert gas, a tried and true method.
If done correctly it's painless, you lose consciousness around 30-60 seconds. But you can struggle with SI before you fall a sleep.