You can’t go too wrong buying from DigiKey in the United States, or Farnell overseas. Be aware that, if you are using someone else’s PCB layout, he or she will often provide the parts specifications using the item codes of his or her favorite vendor. There isn’t an universal UPC or name for parts.
For most parts, the description is the specification. For example, an Epcos B82464G4103M inductor is an Epcos B82464G4103M inductor, ignoring that DigiKey also sells it under the SKU of 495-3461-1-ND and Farnells’ SKU is 742-9444.
Most components such as resistors and capacitors are ubiquitous and nearly fungible. You can swap out manufacturers and minor specs as long as the main desired value (resistance, capacitance, etc.) is fulfilled—and as long as the part has the same form factor. That means “it needs to fit onto your PCB in the holes provided.”
The basic parts you will get include resistors (e.g., 47K 5% 0.125W 1206, the latter indicating it’s an SMT part) and capacitors (e.g., 4.7uF 16V 1206), which are defined by their electronics properties, independent of manufacturers. Diodes have a short manufacturer-given ID (e.g., CRS06, which is a Toshiba part), and if you have trouble finding one, you can often find an equivalent from another manufacturer.
A fair number of parts will be from a specific manufacturer, including sockets (e.g., Tyco 5-1814400-1) and the occasional IC (e.g., Maxim MAX9929 current sense amp, shown in Figure 2-2). ICs in particular can be hard to ...