Welcome Nelly, There is a lot of great information here. One inexpensive place I find parts is the local Dollar General. There is bound to be something like them near you. They often stock a variety of toys that include electronics, plastic boxes, small motors, linkages, tubes, etc.
Most of these toys or other items sell for $3 or less!
If you are looking for some more sophisticated prototyping, two companies come to mind:
Quickparts.com and Protomold.com both can produce parts from 3D models with a variety of processes to choose from.

Scott, that is great, I was wondering the same thing. I do my own non precision sheet metal but I never knew that it was the firs phase….. makes sense since a mould has to eventially be made to produce it in plastic.

One of my projects, I built the prototype with a cig lighter adapter and bolted it to a small bread board box from Radio Shack, used hot glue gun to develope some curves in the shape and mounted an electronic name tag to the front for my display.

I did this to get the dimensions and spacing correct and I ended up with my circuits inside and it was a real working model for about 50 bucks.

The cool part, I did it this way (I thought) because I couldn’t afford to do it the right way. Now I know that basically that is the way it is done. Thanks :-)

Nelly, A prototype plastic case might not be the best option. My industry, precision sheetmetal, is usually the first phase. Going to plastic usually comes in when all the dimensions and revisions have been proven out. Precision fabricators can laser cut and form a prototype in a few hours, have it welded and powdercoated and you will end up with a very nice presentation piece.

just giving this a little nudge up to the top…anyone got any suggestions for Nelly?