What type of wood should I use when building a built in entertainment center?

I’m looking for a wood that won’t sag over time or be extremely heavy. I was told to stay away from partical board (will sag) and plywood (way to heavy). I want to stain the wood, not paint it white or anything like that so I need a wood that looks nice also. I was told to use a solid hardwood but I can’t seem to find ‘sheets’ of wood wide enough to build an entertainment system. Any info (along with websites that sell the stuff) would be appreciated.