There are multiple sites dedicated to add-ons for all of Microsoft's Flight Simulator series. Though most aircraft are specifically designed for one version, they are often interchangeable. An excellent compatibility chart is available here.
You can do this step two ways: if you have a specific airplane you want, you can search the internet or a specific site for it, or just browse a library and see what catches your eye. My personal favorite site is Simviation
, as they have a vast library that is well organized and easy to search. There are other websites out there, such as Surclaro
, that require a free account to be formed, and still others that require you to pay for membership. Most sites give you the file size before you download it, but be aware: as a general rule, the larger the file, the more detailed and accurate the model will be. On the other hand, I've downloaded 4 MB aircraft that looked and performed worse than 900 KB aircraft.
When you download the file, save it to your desktop. It will make things easier later on.