I simply picked up some organic garlic at the store and planted it. I've done both white and purple garlic. It's worked well for me for the past 2 years... And cheaper than buying online.
I planted grocery store garlic this year as well. The organic garlic did very well, the conventional garlic was sprayed with a growth inhibitor so it took too long for the spray to break down before they started growing since I planted late. As long as you plant with plenty of time for the growth inhibitor to break down then it grows too!