Maybe glass beads, pebbles, unusual buttons, coloured sands! Just a few ideas. Hope whatever you make turns out beautiful. Love Chris

If you are near a craft store you can buy a bag of river rocks or maybe if you can't find colored sand get some aquarium gravel. Anything with some weight to it.
Bev

A link to make your own colored sand. I am not sure where you would buy sand. Here you can get it at hobby stores or at one of many naturally occuring sand locations.
I use "Mason Sand" for my pin cushions. It is almost as good as emery sand. Looking forward to seeing what you come up with in the project section.

The jar your using, is it big? Somebody below suggested buttons, could you fill the jar with buttons to tie everything together? or is it to big?

The Dollar Tree has some nice glass beads, sea shells and marbles. Also you can use some dry beans or rice. I'am sure whatever you choose, it will come out beautiful. Hugs, Rachel

To add to the other answers... Yes, I'd put something pretty in the bottom of the jar, for weight and look.
These pincushions sound fascinating.
AlmaG.

Our Walmart sells Plaster of Paris. It can give weight and you can color it to match the background of your fabric. I also like the other suggestions.

You can usually get sand at a pet shop . Hugs Joyce

You can paint (swish it around in the jar) the inside, then fill with anything, old garden rocks! Cat Litter works for sand, and can be poured over the rocks to fill the space. If the jar is painted, the insides don't have to be pretty, just weighty. I would be worried about a glass jar, because pin cushion might end up crashing to the floor, could you use a plastic jar? Linda