This can vary due to buyer agency agreement but in most circumstances the Seller's broker is responsible for the coop compensation.
In my opinion you would want an agent/broker to represent you and look out for your best interests. An experienced broker can answer questions, walk you through the purchase process and negotiate on your behalf.
I agree with sunnyview and you need to find your own agent in the area. An agent will be happy to discuss the buying process with you in a buyer's consultation before scheduling showings.
Can't speak for SoCal but average commission rates in Chicago are 5-6% of purchase price.