Meat trays, deli tray covers, and old baking pans work nice. Old Tupperware... Even old Jello molds
Nice thing about Deli ray covers...you get to eat what came in them!
And you only mix amounts you need. Although it's always best to have something for a backup, incase you have left over mix.

Marcia

My MIL used those plastic trays that frozen pizzas sometimes come in. They were about 12-13" in diameter and about 2" deep. She reinforced the stepping stones with a circle of chicken wire. The textured base of the pizza trays make a nice finished surface. Cheap & reusable.

Cheapest, cardboard box lined with a plastic bag!
If these are for your yard, just cut a shape out of the ground and pour in place. Billie

You might like to try Goodwill or one of those type places. I usually have good luck coming up with some very cheap plastic trays & forms.

Aud

prodgm, you have been giving several good cheap ideas here. Another one I have used is to have my DH cut an old plastic bucket into 3" circles, you can lay these on plastic and fill as deep as you want, or for slightly smaller stones, use round plastic ice-cream containers.
We usually do the 'in ground' method, by using just the leaf, although if the leaf is large this can be back breaking (at least for seniors) as we usually mix by hand in a bucket and do a couple of layers for each big leaf, needless to say we usually only make 2 stones at a session. Also we use a cement mix, not a hypertufa mix. Wendy

Thank you all for your help.