A couple of popular hobby type websites are Sparkfun (https://www.sparkfun.com/) and Adafruit (http://adafruit.com/). You can obviously also buy sensors from places like Mouser or Digikey, but you'll have to do a fair bit of digging through datasheets. The nice thing about Sparkfun and Adafruit is they publish plenty of tutorials for getting stuff running with Arduino or Raspberry Pi.