You don't say "source of livelihood. " The reason is that the word "source" is redundant. If you just say "this is our livelihood" you've said the same thing. The other sentence about making a living from this is fine. One of the finest writers of all time was Bertrand Russell. A native English speaker from Britain, Russell said as a little kid, among reading ever book he could get his hands on, he would go through this little language exercise. He'd take an English sentence and practice re-writing it using as few words as possible without compromising the meaning. It's actually a great little tool to remember. The best writing style (and this is probably true in every language) is saying something you want to say using as few words as necessary to convey the meaning. Needless to say, Russell was a brilliant writer and a child prodigy.