I would recommend not buying raw rock,or nodules,cobbles.I would suggest buying either preforms or spalls.For a beginner big hunks of rock,or nodules,will reveal alot of gravel,due to a lack of spalling skills.I went and got about 500# of rock when I started and man did I ruin some good rock.I would buy spalls or preforms.Good Luck!
There are alot of different places to get rock,knap-ins are good if there are any close enough for you to attend.If not there are places like neolithics,red rock lithics,etc.Also,try the classifieds on paleoplanet.