I SCUBA Dive, My weight belt(s) are perfect for securing light stands, tripods, reflectors, booms etc. You can add or subtract weight depending on how much you want or need to carry. They are flexible and will wrap around your stand or lay flat across the base. You wear them like belts so they are easy (kind of) to carry to the location, put them around your waist, buckle them up and go.
I use soft weights, not the old fashion hard cast lead ones. The ones I use are like bags of BBs so they are less apt to scratch your gear or damage whatever you are setting them on.
I guess you can find them online but in the meantime, stop in at a Dive Shop (not Dive Bar) and see if they might be what you are looking for.