For the costume items whose name starts in "Personalized" (such as the Schoolboy/Schoolgirl outfit, for example), those costume items are gained via the Self-Improvement quest chain initally given to you at about Level 15, the one requiring the Dank Scroll
. I think this questline continues until 60-65, or so? (I don't quite remember what level it ends at as it's been a while for me)
However, most of the items in that quest line are non-statted, or give only a small increase (such as 1% Max HP/MP, -1% MP reduction, etc.), if costume stat bonuses matter to you. They also cannot be dyed - they are set as is.
For other costume items, you can use the methods other players have listed above.