With any type of wood, including osage, you will need to wrap the tip to prevent splitting from the set screw and from heavy use of pressure or percussion. Believe me, you will develop strength fast and will split the wood easily. I recommend that you wrap with steel wire. Most knappers use rods of Delrin or UHMW plastic so they don't have to reinforce the tip.
The hardware stores here only carry copper grounding wire, so that's what I use. I believe the largest size I can get is #6 or 6AWG and is about 5/32" in diameter. I can also get #8 or 8AWG and that's about 1/8" in diameter. It is suitable for glass but not for tackling thick, tough rock. Some hardware stores carry copper rod and copper nails that have larger diameters and are made of tougher copper. The stores in my area DO NOT.
If you do a google search for flintknapping tools and search the images (not the web links) you will get plenty of ideas.
Edit: I'm going to check on the wire gauge sizes. I can never seem to get these right. I just go to the store and point to what I need.... and they cut it.
OK, I checked the sizes. The #6 and #8 copper grounding wire should be available everywhere. The #4 and thicker may not be available unless you go to a contractor's supply store.