Unlike all the other languages that can be used in a web browser, there is no need to download and install JavaScript because those browsers that support JavaScript at all have it built into the browser where it is turned on by default (meaning that you need to change your browser settings if you don't want the browser to run JavaScript). The only exception to this is that Internet Explorer also supports vbScript the same way and the two languages are controlled via settings called "active scripting" rather than referencing JavaScript specifically.
What you will need to download with JavaScript then is not the scripting language itself but rather those scripts that you want to run in your web page (assuming that you have decided not to learn JavaScript so as to write it all yourself).
There is no need to pay for scripts written in JavaScript since just about any script that you can imagine is available from a site somewhere as a free JavaScript download. What you do need to be careful of when you decide to get scripts for your site though is that you download them fron a site that is actually offering the scripts as a free download rather than just copying the scripts that you want from any site. JavaScript code that performs any significant task will be subject to copyright and you will therefore need the permission of the author to use their script. Of course what the javaScript is doing is not able to be copyrighted and so if you are writing a script yourself you can't get into trouble for looking at an existing script to see how they did it and then writing your own version but if you are just looking for free JavaScript downloads to use with your site without having to rewrite it completely yourself you should go to a site where the author specifically states that their script is available as a free download and can be used on your site.
There are many sites that offer free JavaScript downloads including several huge sites where that is all the site offers as well as other sites (such as this one) which both offer free JavaScript downloads and also have tutorials on how to write the scripts for yourself. Apart from avoiding copyright issues there are further advantages to getting your free JavaScript downloads from a site that is actively offering the scripts, the main one being that such sites usually also provide you with step by step instructions on how to actually install and use the script. Where you just grab a script from anywhere not only are you stealing a script without permission but you also get no instructions on how to install it and no support if you can't get it to work.
Another thing to watch for when you are getting your free JavaScript downloads from an appropriate site is that many of these sites have been in existance for many years and the way that JavaScript should be used has changed somewhat over that time. In many cases there are scripts still on offer that were written many years ago that were written the way that everyone wrote JavaScript at that time and which were written to work best with the most popular browsers that were available at that time. Often those scripts are still available for download today even though the browsers that they were designed to work best with are long gone. You really need to get at least some familiarity with what is currently considered to be the best way of writing JavaScripts in order to be able to choose the version of the script that you need that uses the most modern approach and which will therefore work best with current browsers. You can't blame the sites offering scripts that are not right up to date because it does take time to code and test JavaScripts to make sure that they work correctly with the wide range of different browsers in common use today and having a somewhat dated script that still works available as a free download is certainly a better alternative to not having the script available at all. The biggest sites specialising in free JavaScript downloads are usually offering scripts written by a lot of different people and they are reliant on someone actually writing a new version of a script before they can offer it for download. Other sites that have their own staff to write and test scripts may have limited resources for being able to create updated versions of scripts and so may be offering older scripts until such time as they can revisit the script to update it.
