I think you are right, if you are not happy with your therapist I don't see a problem with checking around before you make a decision to terminate with your current therapist. You might end up finding that your current therapist is a lot better than the available options (clearly not your case).

Tell T2 the truth, that while T1 was unavailable you made the decision to check around a bit, with T1 gone not like terminating was an option until he/she got back. And when you make a final decision you will inform T1. Do think letting T1 know you aren't coming back for whatever reason is the polite thing to do but you are under no obligation to do so.