If your product idea is complex and way beyond your level of experience, you may have some difficulty creating a prototype--you may even need to hire someone to make it for you. But more often than not, you can make your own prototype if you're willing to roll up your sleeves and learn what is needed. Compiling the research to understand what goes into making and designing your product will help immensely when you start selling it.
Begin by writing down all the materials, supplies and tools you may need. Next, try to identify the various steps required to assemble your prototype. Creating a prototype is like building a model airplane: You don't make the body and then install the engine. Think about how each part works with the others and how each phase fits into the next. This exercise will be invaluable when you have your product manufactured.
Now identify those parts or materials you may already have around the house or can purchase easily. I've cut off the bottoms of prescription bottles and used the sides for tubing. I've removed spirals from notebooks because I needed a wire coil. If you're having trouble coming up with certain parts, think about what items can substitute for the missing parts. I had an idea for a pool gadget that required a waterproof motor. I found my answer when I went to a pool supply store: a battery-operated pool toy that had a motor inside.
If you can't find the part at home, the next step is to buy it. Some places worth exploring are hardware, grocery, fabric, craft, computer, building material, beauty supply, kitchen supply, and toy stores.
When building a prototype, don't try to reinvent the wheel. Keep in mind, right now you're just trying to get a working model of your idea, not the final production model. For now, make it easy on yourself and substitute.
In most cases, use standard parts. They're always easier to find than custom parts and will be easier to purchase in bulk when the product is manufactured. It's also easier to adapt parts for your prototype from existing products (such as radios, bicycles and so on) when you use standard items. Keep in mind, however, that if you can easily get standard parts, so can your competition. Using a few custom parts creates a barrier to entry for your competition.
Take your time when making a prototype; it will be more effective if you're patient and think through each step. Several good things can result from proceeding slowly and carefully. First, you may discover a change that could make your idea work better. Second, you may realize there are parts that you forgot to get or didn't even know you needed. And third, your frustration level will be reduced while you go through the process slowly because you will know that you thought each step out carefully and did not miss something by rushing.
Remember, the goal of a prototype is to prove your idea works. There will be lots of experimenting and tinkering. Don't become concerned over the material costs at this time. Right now, you are creating the most expensive version of your idea. Costs will get lower as you finalize your idea to its most efficient form and can reduce labor and buy materials in bulk at wholesales prices.