Wordpress plugins source code is published using GPL licence, this means that if you found the source code, you can use it. However you won't have commercial licence, which means no support, usage at your own risk. An easy way is to find a website that is using the plugin, and argue the webmaster to release you the GPL source code for free.
If you can provide some other plugins with fresh updates it will help, you may consider buying one update, and distribute it without the commercial licence in exchange of other plugins.
Some websites are selling at very low discounted prices with this technique.