Since version 2.3.4 plugin is available on Atlassian Marketplace as Paid, with price $10 - $600 depending on number of users in your Jira.
Note, it is still open and free for open-source projects. You can request free open-source license similarly to Atlassian products here, see Marketplace Add-on option.
However, it's not possible to buy it directly in a few countries (mainly India) as described here. Please, contact one of the Atlassian Experts listed on this page instead, who can help you facilitate the purchase.
You have to buy license for continued use. It includes 12 months of maintenance. Renewing license is at 50% of the current purchase price, and is optional and needed for receiving updates.
Also, there are few functionality restrictions with no valid license. You'll see UNREGISTERED all the time and few things will not be available:
- Plugin Configuration in Administration -> Add-Ons
- Subscriptions to Timesheet Report), to receive it by email
- Worked Time Chart
- Timesheet Holidays
- Plugin updates notification.
Note, you can ask for help, vote or request new features here or via email, disregarding license validity.