Options;
1) Ikea silicone ice cube trays
2) Silicone chocolate or candy moulds (ebay works)
3) PM Hobbycraft, Micheal's etc. also carry small moulds for things like trees and trains for model building. Sometimes you can find flexible containers to make rows upon rows of items.
I've used all of the above successfully for at least 5 years and counting. I currently use a whole pile of silicone chocolate trays I bought from ebay. They were like $5-6 each and make a variety of small shapes. I have ones that hold about a teaspoon all the way up to ones that hold a couple of tablespoons. Like
this.
For a couple of years my fish ate their frozen food in the shapes of stars and hearts.
Good luck!