I recommend using the method that everyone else uses.
Contact all the suppliers you know and ask for prices. It shouldn't take you very long to find out who charges the most, and who charges the least.
To presume your local supplier is ripping you off is possibly short-changing them, because often, like in my neighborhood, a local shop buys enough to pass his savings along and provide you convenience, and him a modest profit.
There's a plethora of suppliers online too, and the prices are easy to see.
When you start buying in any kind of quantity, you'll start getting the some buying power, and that will change the whole picture once again.