While the first tuning is in the Cartel Market, there are plans to add more tunings to the game available from a variety of sources, including straight gameplay. The tuning is also not any more rare than other items within a given pack. There are also plans to add slots to more items that don’t currently have the tuning option, so if your preferred weapon lacks the option, that may not be forever. The answers may not mollify everyone, but they do at least address concerns.
Source: Official forums