These are also the same as baking molds. If it’s silicone and it’s labeled for candy or chocolates, but it works for your project- then go for it.
Any leftover container from food or beverages. Bowls, cups, Tupperware, sour cream, yogurt, leftover takeout containers from restaurants.
Balloons are an incredible tool to use as a cement mold.
You can cut open just about any type of plastic ball and use it as mold as I did with this Round Cement Christmas Candle and these cute little DIY Magnetic Cement Planters.