Summarize:

Developer is often sold with hair dye in a package, so you don't have to worry about choosing the right volume. Getting them together is ideal because the developer will be the right strength for the color on the package. However, keep in mind that the hair color will likely look different on you than it does on the model on the box. Your hair color will most likely turn out a shade or two darker or lighter than the image on the box. If you bought a lone tube of hair dye, get the developer separately. Choose the developer volume you need. Buying developer separately may help you achieve the desired results.\  If you bought boxed dye with the developer included, it is not a good idea to buy separate developer. Use the developer in the box for the best results. You may want to purchase the same brand to of dye and developer to ensure that they will work well together. Running out of developer and hair color when you are halfway into the dyeing process can cause your hair to come out uneven or improperly dyed. Avoid this by getting an extra box of developer and hair color so you have it on hand. A good rule of thumb is to get at least 2 to 3 boxes of hair color and developer for long hair (past your shoulders) and 1 to 2 boxes of hair color and developer for short hair (above your shoulders).
Look for hair dye and developer sold together for an easy option. Buy the developer separately if you bought a tube of dye. Purchase more developer and hair color than you think you might need.