Game stores often carry tubes of flat color beads that I've used for this purpose.
Another great thing to do is:
1. find an image of the critters you want
2. copy the image to your hard drive
3. open up MS Paint and crop the head of one of the critter in roughly a square
4. open up MS Powerpoint and paste the head of the critter on a blank page
5. with the image "active" (meaning you've clicked on it) go down to the drawing tool bar and click on the icon "line color" - this will put a black line around the image, making it easier to size and cut out
6. resize the image to be approx. 1" x 1"
7. copy the image and then "paste" as many of the critters as you need!
8. print and cut them out - instant army!
I often use Fiery Dragon's counter sets for this. When I like to get fancy I use spray adhesive and glue the printed page on some tougher card stock, and then cut them out for tougher counters.