Learn Farsi: از فروشگاه لباس می‌خرم.

In Farsi, lebās is usually treated as a general concept or a mass noun when talking about clothing. You do not need to add the plural suffix -hā unless you are referring to specific, individual pieces of clothing, like pointing to a pile of shirts and pants.

Why isn't there a rā after lebās since it is the object being bought?

The direct object marker rā is only used when the object is specific or definite, such as "the clothes" or "my clothes". Since this sentence refers to buying clothes in a general, indefinite sense, lebās does not take rā.

How does the word order work here? Why does az forūshgāh come before lebās?

In Farsi, prepositional phrases like az forūshgāh (from the store) are flexible but often come right after the subject. Because lebās is an indefinite object without rā, it prefers to sit as close to the verb as possible, naturally pushing the prepositional phrase earlier in the sentence.

Is there a trick to remembering the word forūshgāh?

Yes! It is built from forūsh (the present stem of the verb meaning "to sell") and the suffix -gāh, which means "place of". So it literally means "place of selling". You will see -gāh in many other location words at this level, such as dāneshgāh (university) or forūdgāh (airport).