Answer:
Nosotros te los compramos.
Step-by-step explanation:
In Spanish, the words "te" and "los" are direct pronouns, and represent a replacement for both indirect and direct objects. The direct object in the original sentence is "los boletos" (the tickets), and the indirect object would be "ti" (you).
To be clearer:
Nosotros compramos los boletos para ti.
[subject] [verb] [direct obj.] [indirect obj.]
Is replaced by:
Nosotros te los compramos.
[Subject] [indirect obj.] [direct object] [verb]
Thus, "te" corresponds to "para ti" and "los" corresponds to "los boletos".