A wide variety of sensors can feed info to the Raspberry Pi. Some of these, such as the Raspberry Pi camera, can be directly connected to the Pi. Others are perhaps best connected via an Arduino, with low level processing done there and the results streamed to the Pi through a serial port (either on the GPIO pins or via USB). 

Using an Arduino also allows you to leverage the many Arduino driver libraries that the Maker community has written for a wide variety of sensors. 

When buying in quantity, we have found AliExpress http://www.aliexpress.com/ to be a good source. Think of it as a cross between eBay and Amazon, but specialising in direct sale of quantities from China. If you buy all your components from the one vendor, you can combine postage. Shipping can be problematic, in our experience DHL has provided reasonable service. 

Stores that we have had experience with are as follows. These should not be considered endorsements or any suggestion that other stores are not worth checking out, these are simply the ones that we've ordered from in the past, received the goods in a timely fashion and found that they matched what we ordered. 
  * http://www.aliexpress.com/store/116008 (mainly Raspberry Pi cameras and cables)
  * http://www.aliexpress.com/store/612195 (all manner of sensors, Arduinos and prototyping bits and pieces)
  * http://www.aliexpress.com/store/1051119 (smart LED pixel strips and supplies)