There are a number of situations where one can be lenient to purchase new clothes in the 9 days (great financial loss, shidduch, for a mitzvah). It is up to you, and/or in consultation with a Rav, whether this is applicable to your situation. The poskim of the generation were forthright in the importance of the days leading up to Tisha B'Av, but they were also aware that in exceptional circumstances one can be lenient.
We cannot know your personal situation, so this is a subjective matter. For example: if a person has been searching for a job for a year and has 7 mouths to feed, only a Rabbi disconnected from reality will advise you to 'take the hit' and not buy new clothes. Ideally, you'll find a middle ground, but if one can't then there is room to be lenient.
Rabbi Melamed writes that for the purposes of a mitzvah you are allowed to buy things that bring you joy. The examples he brings are tefillin and sefarim as people generally don't say a shehecheyanu on them. He then goes on to say that if you dont have (basic) shoes for Tisha B'Av you can buy them b'diavad.
The Mishna Berura (551:13) writes that in a case of great loss (e.g., fixing a wall that is falling, even if it gives you simcha), you can fix a wall.
If you have a wife and kids, a Rabbi might extend this to buying respectable clothing that doesn't require a shehecheyanu (i.e., it doesn't need to be a 3-piece suit) for an interview. This is because it is a mitzvah to support your family and provide shalom bayit. However, if you are a 17 year old looking for a summer job, this may be a different situation.