Instead of buying the $100 freezable roller, place a water bottle in the freezer and roll the bottom of your foot with it. You can experiment with bottles of different shapes.
Tennis balls are great muscle massagers. You can use them pretty much on any body part, it's cheap, and portable. This case was $.99 at the 99 cent store. Roll the bottom of your foot with it. Really put your weight on it to get a deep massage.
Place the ball inside a knee length panty hose so you can control the ball from rolling away. Also a great use for back massaging. Place the ball on your back while standing against a wall. Press and hold the ball on those stubborn muscle knots.