Notes

#1.Frame / Platform:Wood: Depending on the size of your robot, plywood may be a good choice. Plywood is inexpensive (or free) and easy to work with using simple tools. You can get 1/4″ and 3/8″ plywood at Home Hardware, Home Improvement, Craft, or other home improvement / construction / hobby stores. You should also be able to find a lot of free wood locally. For example, a cabinet shop may throw out a lot of wood which is too small for their use but can be perfect for building a robot frame.
Aluminum: This material is light weight and moderately easy to work can also be found at Home Hardware, Home Improvement, Craft, or hobby stores. You should always be very cautious when working with aluminum since the edges can be very sharp and should be sanded or filed to round them. The thicker the aluminum, the harder it is to form using hand tools.
Plastics: Acrylic or Plexiglas are both easy to work and can also be found at many Hardware, Home Improvement, Craft, or Hobby stores. High-speed tools should not be used since they will melt the plastic. When cutting or drilling use low speeds.
Re-purposed Materials: There are a lot of items around the house which can be turned into a robot frame. Take for example and old CD; These can be easy to find most people get them free in the mail from AOL or Earthlink. Instead of throwing them away you can use them to make a robot. They can be a little on the brittle side so go easy when you do any cutting or drilling.