How do I get bitcoin price without any external source? I have searched the Internet for solution but could not find the one I was looking for. 

I want to get bitcoin price (in usd) without using any external website (meaning not getting the price using api or something) 

I want to "calculate" it myself, since there are websites who display their bitcoin price and it's slightly different from other websites I guess they found the price from their own. 

How can I find the bitcoin price by myself?
The bitcoin price is determined by a free market of buyers and sellers. There is no way to calculate the price on your own without an external source. I suppose the only exception to this would be if you ran your own bitcoin exchange and you had enough users and volume to produce a fair market price.