WordPress Plugins are open source and you have the right to get them from the best available source. Don’t take our word for it, listen to the experts:

The license under which the WordPress software is released is the GPLv2 (or later) from the Free Software Foundation. A copy of the license is included with every copy of WordPress, but you can also read the text of the license here. […] Plugins and themes are derivative work and thus inherit the GPL license –
http://wordpress.org/about/license/

And if you decide you need support for your plugin purchase, you can purchase it directly from the plugin author.