Welcome, Dylan Auty!
I also advise against using golf balls or any other hard object, when you are beginning. Tennis balls are much safer. People have also made low-impact sling ammo out of paper and duct tape, or yarn, or other soft materials.
The first few slings I made were braided.
Dan Bollinger
and
Victor Escobar
have written very good instructions on how to do it, starting with nothing but twine.
And as Dravonk mentioned,
AussieSlinger
has offered sling pouches to any who ask (you supply your own cords). Just send him a private message, introducing yourself and giving your mailing address (do not post it here, the forum is not for private information).
I am not sure how to help you on places; I like parks myself. An unused baseball, football, or soccer field are good places to sling. I have also been known to bounce tennis balls off the side of a building, when I can find a large brick wall with no windows next to an open area like a parking lot.