Making Your Own
For some games, it's best to make your own playing pieces. For example, for "Space Battle" I used hexagonally-shaped pieces of colored paper (red for one side, blue for the other) that have star-ship images and names printed on them. I cut these out so that each figure fits on a playing "hex."
If you don't feel like creating your own images, you can simply search for them online. Or, you can skip the images and use other ways of identifying the different kinds of pieces, like a special font (which, again, can be found online).
Your child may also enjoy creating his or her own pieces. This generates a sense of pride and ownership. As a rule, children love to play games that they've helped to create. So, the more you can get them involved, the better!
Other ideas
If you own some board games, you can use pieces from them in your own game, especially if you're no longer interested in playing them.
Toys that you already own also work well. My son has many Lego and Playmobil sets; these come with nice-sized figures that make good playing pieces. For "Race to Altair," for example, we often use Playmobil people from one of the "space" sets, while we use knights for "King Richard's Ransom."
You can also buy inexpensive plastic figures to match your game's theme. These can be found on Amazon.com or on ebay. (You just have to make sure that the figures are the right size for your game.) There is a company that sells tubes of 10 painted plastic figurines for about $7.
For a little more money, you can buy wonderfully detailed miniatures from a game or hobby shop. If you don't have one of these near you, you can look on ebay.
Using figures that your child will like playing with is a great way to generate enthusiasm for your new math game!