Starting off at low weights, you can use something simpler. Water. Just use a plastic milk container (if they come with a handle this is useful), a coke bottle or something, fill and weigh. Increase the weight slowly, rather than in large steps, use two milk containers to increase further. Also there are no questions about what the weight is for if stealth is a factor.
The only issue with water is the bulk, which will become even more of an issue as weight increases.
An alternative is lead buckshot or fishing weights (if they can be purchased cheaply). Again you can modify the weight to the nth degree but bulk is less of a problem. Also you don’t need a sealed container, so a jug would do. In fact you could use anything made of lead as long as you have a receptacle you can fit it into.
Almost anything can be made into effective weights when you start hanging. 1kg bags of sugar in a carrier bag. Anything with weight :)