Here’s an old prop chestnut which we’ve used during our current production of Bloody Bloody Andrew Jackson. Let’s say your play requires a switchblade or two. Real switchblade knives are fairly illegal; even if you can find a place that sells them, you often need a special license to purchase it. However, you can buy switchblade combs at many novelty stores.
You can fill the gaps in the comb part with Bondo (or whatever your preferred brand of auto body filler is). Sculpt it into the shape of a blade, but be careful not to get much thicker than the comb; if you add too much thickness, the blade will no longer pop out.
Finish it off with silver-leaf. Again, silver-leaf will add no thickness to the blade (which some paints might), and it will give it the ability to glint and gleam from the stage. It will also be less likely to rub off during frequent usage.