Gold and Silver are immediate precious metals that spring to mind when it comes to this subject. There are others though - Lead, Copper and Iron have their uses as well.
Gold, while expensive is compact and never loses its luster. While silver tarnishes slightly both have a track-record stretching bag to pre-history as a medium of exchange. Also silver, unlike gold has a degree more practicality with excellent conductivity. Gold however is an excellent dental aide and is very difficult to forge (although tungsten is an issue with some large bullion bars).
In a truly catastrophic TEOTWAWKI even precious metals like gold and silver will be much less desirable. Even the most hardened of gold-bugs is forced to admit that as wealthy as gold is, it's difficult to eat gold!
Overall though, it's best to have a mixed bag of precious metals in your prepping stores.
Gold and silver are available online and in Pawnbrokers. Jeweler stores and Pawnbrokers are best if face-to-face transactions are your preference. The price of gold and silver can vary, you can keep up to date with the prices by checking the charts off to the left.
Lead used to be greatly available from tire centers due to the amount of wheel weights they would end up with over time. However in recent years zinc has been replacing lead - making cheap lead sourcing from garages to be hit and miss. You can melt lead easily, it has a low-melting point (although the toxic fumes must be prevented from entering your lungs). Zinc's melting point is higher making it a way to identify the two metals by temperature.