For learning purposes free is good but you do run the risk of fighting with a blank that has "issues".
For my own cheap self I thought that built character and taught me a thing or two along the way.
If you're not as accomplished at rationalizing your frustrations then by all means purchase some to get an idea of how fun it can be.
Wall lumber usually has a few pallet loads of ambrosia maple blanks at what to me looked like a fair price.
You can check out their website,
http://www.walllumber.com/turn.asp
As for green vs dry
Green is a joy to turn but you need to rough turn the blank, dry it, then finish turn.
This is not a quick process though Ive seen others with tricks to speed the drying stage, I have no experience with that.
I put the rough turned blanks in a paper grocery sack along with shavings, write the wood species and date on the bag and check back in a few months.
Turning dry wood is not as quick but cuts out the drying step. Depending on species you may get a lot of practice keeping a your tools sharp.
Good luck, I was in your shoes a few years ago and have thoroughly enjoyed the journey. Folks here are full of good advice and if you can agree to a time there are a few who will give you priceless one-to-one hands-on tutoring.