If you have decided to build a piece of furniture you might find it hard to find the right materials at a price you can afford. This means you will either need to search in unconventional places or be prepared to spend much more than you might for a premade piece.
Today the cost and availability of hardwood is not what it use to be. In your average home store you are only likely to find small widths of hardwood used to modify or repair premade cabinets. You may also find laminated plywood that has a hardwood veneer. If you want solid hardwood of larger widths you will need to contact a small supplier that has access to a wider range of sources.
Some people who enjoy woodworking as a hobby will actually harvest their own lumber. Where I live there is a mobile company that will come to your location and mill your logs in to boards. There is a pretty good fee for this service but it allows you to define the exact measurements of the materials you need.
Another way to obtain raw materials is to get them from recycling. Often you will see people put out furniture for the trash. This is a good source for wood.
You may also see wood pallets available for pickup but most of the time this wood is subgrade quality and contains preservatives that make it impossible to hold stain.
Final Note
Obtaining materials has become more difficult lately but with some innovative planning you should be able to get what you want with reasonable cost.