Having trouble finding ammo for the sling i have. Never seems to be in stock in Hommlet and never seems to be part of the booty. Any suggestions?

Search the barrels in Hommlet. There should be some in those. As well, the Hommlet merchants restock after 1 week, I believe, so rest and shop. I think both the smith and the leather merchant carry them (it may be the masterwork leather merchant that has them - can't recall for certain). If all else fails, you could use the in-game developer console to give yourself some.