Either you own 21's embedded Bitcoin device which can mine Bitcoins itself or you buy them from one of the myriad of exchanges.

http://circle.com or http://coinbase.com for instance.

Also, "The easiest but most limited way to get some bitcoin with 21 is to run 21 faucet, which gives you a small amount of bitcoin to play with."

This service actually comes with a way to get bitcoins for "free" or mine with their hardware.

"The easiest but most limited way to get some bitcoin with 21 is to run 21 faucet, which gives you a small amount of bitcoin to play with."

https://21.co/learn/intro-to-21/#21-faucet-get-bitcoin-from-...

Coinbase.com, Bitmarket.net, Kraken.com to name a few exchanges.

Also, http://www.reddit.com/r/btc is a nice bitcoin community.

Alternative opinion: /r/btc is full of cranks and conspiracy-theory credulists.