I'd like to do some practice casting before I take my son out with his rod and reel again. Can you buy rubber weights/leads to attach onto a line? Or is there another obvious alternative? I'll be practicing on an astro turf pitch.
As @Dave said, rig up as though you're casting normally, be it float or feeder (minus the hook for safety). You won't get a 'feel' for the tackle used, or get the accuracy using substitutes on the line. If it's accuracy that you want, get a box or something like that and try casting into it. But rig up as normal.