Question

1. Where do you buy a scarf?
-I buy it at the department store.

2. Where do you buy the scarf?
-I buy it at the department store.

3. Where do you buy scarves?
-I buy them at the department store.

(Which question is correct grammatically?)

Answer 1

All are grammatically correct, but only #3 is the smooth and commonly used expression. To make #s 1 and 2 better, here's how I'd rephrase them:

Where do you go when you want to buy a scarf?

Where do you go when you want to buy the perfect scarf for your mom?