Many countries produce petroleum. Some of the countries are Nigeria, Saudi Arabia, Russia, USA, Mexico, Venezuela, Iran, Kuwait, Iraq and the rest of other countries. Your first step will be to find suppliers in these countries. It will not be easy to connect with crude oil suppliers in Iran due to the fact that they are heavily under sanctions put in place by USA etc. You will need to read the terms of the sanction before you think of buying from Iran.
USA produces oil but it imports as well. As such, you may not easily find a place to purchase petroleum in USA because demand exceeds supply. A good place to easily find sellers of petroleum is in Nigeria. Nigeria is the largest producer of petroleum in Africa. It drills millions of barrels in a single day. Since the ethnic violence which usually occurs in Nigeria reduced to its lowest level in 2009, more and more barrels of crude oil is produced on daily basis. Sellers of petroleum can easily be found in Nigeria due to there are fewer restrictions guiding the purchase of petroleum.
Your first step will be to connect with a seller. Once you have connected with a genuine seller, then you can work out the deal with the buyer. Allocation of petroleum can also be gotten from a government agency referred to as NNPC. It is the main seller of petroleum in Nigeria. Allocations can be gotten easily when you go through a broker. This is so because they know the in and out of the petroleum business within the NNPC. Nigeria produces petroleum more than it uses, leaving surplus petroleum available to sell.