A costume store is an establishment that sells costumes, masks, makeup, etc. that people use to disguise themselves or dress up as someone else.
Positive: Dreaming of a costume store could represent childishness or neutral entertainment. Sometimes people dress up for fun (i.e. kids playing dress-up).
Negative: Being in a costume store in a dream could represent one who is under strong delusion from the enemy. It could also represent deception and that a person is being deceived. It could symbolize that one does not know their identity in Christ. Seeing someone put a mask on in the costume store could represent one who is unstable and deceptive.
(1 Timothy 4:1-2; 1 John 1:3; 1 Corinthians 13:11; Matthew 18:2-3; 1 Corinthians 6:10; 1 Corinthians 10:13; 2 Corinthians 5:17).