It depends on the product and what you need the prototype for. Here is a good general Wikipedia article on prototypes: http://en.wikipedia.org/wiki/Prototype
You really need to know what is the purpose of the prototype. Start with the end in mind. A prototype is usually made to prove out the concept to make sure it works before committing to a production manufacturing process.
I’ve made prototypes out of cardboard, wood, or from parts of other products. I then move to more refined models called SLAs http://en.wikipedia.org/wiki/Stereo_lithography. If the part I was working on is injection molded, I would get an aluminum prototype tool made for about $3,000. If the sales volume was high enough, it would justify a production tool.
Can you make a prototype yourself out of household materials?
Can you get an illustration made with a program like Adobe Illustrator?
Do you need a CAD model give the proper detail to communicate the idea to a licensee?
Things to consider:
Does the cost of the prototype include design engineering or just fabricating the prototype?
Working with a domestic company is usually easier; you are probably close to the same time zone so you can call if you have questions and you’ll get your parts faster.
Now to answer your questions:
1. Does that put me at risk for my idea to be stolen and manufactured In China, since my PPA only covers me in the USA ?
In my opinion, you are at potential risk. You can ask the company to sign a non-disclosures agreement (NDA) but as for me, I would not take a chance to save money.
2. If it's a good idea, does anyone how I can go about having some made? I have one product that would be made of steel and others made of plastic.
Short answer: If it is made of steel or some type of metal, a prototype part may be machined out of a block of metal depending on the shape of the part. If it something thin, then a sheet metal part can be cut and bent into shape. I can refer you to some domestic companies I’ve worked with that make small quantities of prototype parts.
For plastic you may be able to machine it from a block of plastic. A rapid prototype can also be made. It’s a process like a 3-D inkjet printer, where the part is made one layer at a time.
I can refer you to some domestic companies I’ve worked with that make prototype parts.