What type and where to get fill sand for my yard?

Looking to fill in some areas between tree roots in my St Augustine grass. I'm located in Slidell.

I've tried mason sand before but was left with small gravel after the sand settled. I've heard horror stories of weeds being brought on with river sand. What type should I use to ensure my pristine lawn is not damaged?

Also, if anyone in or near Slidell can recommend a place to order it that will help.