This takes careful experimentation with parts made of different materials, all which can be very costly when paying full retail price...especially if you don't know if it is going to work or not. Raw materials such as plastic, wood, metal, as well as other varying materials can be bought at hardware stores, lumberyards (for wood obviously) and specialty plastic and metal resellers. When experimenting with a possible material, it might take one quick test to find out that this is not a good material for your desired part, and suddenly that material is non-returnable (and another item to jack up your end-of-year R & D expenses).
Deep discount retailers such as Dollar Tree, Family Dollar, and 99 Cents Only are all great options for obtaining inexpensive materials without breaking the pink porcelain piggy bank. These stores are normally known for helping frugal Mom's buy their kids inexpensive birthday party favors, as well as household items for the kitchen, cleaning products and many other things. But these stores are also an excellent place for an inventor to find the right material to make a part, all for around a dollar. Perusing the isles, one can often find most if not all of the parts required for an initial prototype attempt. If the material is not a good fit, you're only out a buck instead of $10 - $20 for a custom piece purchased through a specialty materials reseller.
You won't find too many items with Made in the USA markings on them. You will find rubber, many types of plastics (soft, hard, flexible), metals (aluminum and harder metals), as well as paper by-products (from thin papers all the way up to poster board and cardboard). Wood dowels (flags), plastic and metal tubing (summer toys and mops, respectively) can all be recycled from anew and fabricated by you to make the parts just right for your project.
Other inexpensive options include Goodwill, St. Vincent De Paul's and other charity type organizations that sell back used items to consumers. These are also good for picking up consumer products are already similar in shape to your new product, have parts that are similar in shape to what you need or can be taken apart to get the "innards", including motors, gearing, rechargeable batteries, etc. Stores such as Wal-Mart and Big Lots carry new items of good quality that might have that specific part you need, without being a killjoy to the end-of-the-month business credit card statement.
Hopefully now you have been able to prove your excellent concept* using the inexpensive parts made from cheap products. After you have proof of concept, you should now move to the next step in the prototyping process... making the best working prototype you can with all the materials you have at your disposal, so it can be seriously reviewed by a potential licensee. Ordering specific materials and parts off the internet can have it at your door within a week, faster if you want to pay more postage.
All in all, you should try to prove your concept utilizing parts made from inexpensive items... but spend the extra dime to get the best parts for your final working prototype. The closer you are able to close the gap between your initial idea and what the eventual product will be manufactured out of, the closer you are to obtaining that license contract with your licensee.
* Some projects may require a fully engineered rapid prototype in order to achieve proof of concept