There are over 60,000 pharmacies in the US, mostly in large brand drug stores and supermarkets. The largest drugs store brands are CVS, Walgreens, and Rite Aid. The largest pharmacy brands in markets and club stores are Walmart, Costco, and Kroger.
There are also thousands of small local pharmacies. Many specialize in compounding and less common medications. You're never far from a pharmacy.
The cost of a prescription depends on your insurance coverage, the pharmacy, and the pharmacy benefit managers that negociate the prices. You have to give a pharmacist your prescription and health insurance information to check your out of pocket costs. Many health insurance plans have separate prescription deductibles. There are some drug company websites and coupon websites that have discounts for specific drugs or pharmacies.
New medications with a patent can only be sold by the brand that owns it. Patents expire after 20 years and then other companies can manufacture and sell the same drugs. These are called generics. When generic drugs are available, they are less expensive. You have to request the generic because people still refer to the drugs by their brand name.
The most common drugs purchased are over the counter, meaning they don't require a prescription from a doctor, nor dispensing by a pharmacist. This includes all the boxed and bottled drugs in the aisles for pain, cold, headaches, fever, sinus, allergies, upset stomach, heartburn, acid, etc. Popular over the counter brands include Advil, Tylenol, Aleve, Mucinex, Benadryl, NyQuil, Pepto, Mylanta, Pepcid, etc. These are carred by every major drug store and supermarket.