One of Joomla's significant advantages is the ability for third parties to create extensions for Joomla. This has resulted in thousands of extensions of good quality. Some extensions are of course of better quality than others, and some have a limited user because of expansions for small niches.
Either way, as a user of Joomla select in an ocean of good extensions. However, be aware that the upgrades and especially larger migrations of Joomla platform can present challenges if you have become dependent on extensions that are not updated by the publisher to fit the new version of Joomla. A good rule to follow is thus not to install extensions uncritically if they are not required to develop your website as you wish.
The official list of Joomla extensions is available at http://extensions.joomla.org. Here you will find both free extensions and extensions you have to pay for. Most extensions however, you can either try when you download a limited trial version of it with limited capabilities, or you can download a trial version that can only be used only for a limited time. There are also many other opportunities to give you a good idea of the program before you buy it. It mainly running again that you rarely have to buy a program without having the opportunity to get a good idea of how it works first.
For most normal needs, there is almost always a good free alternative among extensions. So to keep costs to a minimum, you should limit the use of commercial extensions to the cases where you have special needs.