The proper article depends on whether you are referring to a generic item or a specific, already-identified map:
"I want to buy a star map for my anniversary". buy star map
"I decided to buy the star map we saw online". The proper article depends on whether you are
(Definite Article): Use "the" if you are referring to a specific map you’ve already discussed, such as one customized for a certain date or a particular brand. Quick Reference buy star map