MarAthena
Stop by one or more local bike shops (LBSs) not big box stores like Walmart or Target. Look for stores with brands like Trek, Giant, Specialized, Raleigh, Fuji, and Surly. Most will have used bikes that were traded in. They are probably your best bet for assuring that you actually get what you need at a fair price. If there is a bike co-op in your area (usually a volunteer organization that rehabilitates old bikes) they are also a good possibility. If you are going to buy from a private seller, take someone with you who knows a bit about bikes and what to look for in size, quality, and condition.