Touchscreen LCD alternative?

Not sure if this is right section for this question, so feel free to move it!

I know there are Touchscreen LCDs out there that are $$$, but I'm curious if there is a product sold that's like a thin, transparent sheet that sits on your monitor screen (almost like its taped on, but not sticky of course) and has sensors that allow you to use a matching pen-like device with it so that it gives your regular monitor into the functionality of a LCD touch screen?

I imagine that shouldn't be something that's all too difficult to make, so I probably just didn't know what words to google for it. Anyone know of such a thing?

Thanks!

Try searching for Touchscreen overlay.

http://www.touchscreen​s.com/products-addon.html

I used them before in an old job and they aren't that responsive but still get the job done and save on the £££.