I purchased the one above on ebay for $20 and free shipping. I needed one good one so I could record the sounds for the film sound track. It was the best deal I could find. ebay buy it now link
One that I would really like to get is one that is also a calculator. Like the one in the picture on the right. Again it doesn't need to work because I looked them up and they run from $39.95 and up.
We can't use the black plastic one because they weren't popular until about 1985 I think it was. Silver or gold are best.