Learn Polish: Musimy nabyć coś od nowego dostawcy. - We have to acquire something from a new supplier.

In Polish, when you get, buy, or borrow something from a person or an entity acting as a person (like a supplier), you use od followed by the Genitive case. You would use z if you were taking something out of a physical place or container.

The word for supplier is dostawca. Why does it change to dostawcy here, and why does the adjective nowego not match that feminine-looking ending?

Dostawca is a masculine noun that happens to end in -a. Because of this, it declines exactly like a feminine noun in the singular, which is why it becomes dostawcy in the Genitive. However, any adjectives attached to it still follow standard masculine endings, giving us the regular masculine Genitive adjective nowego.

Could I just use kupić instead of nabyć?

Yes, you could. Kupić specifically means to buy, which makes perfect sense here. Nabyć is a slightly more formal or broader word meaning to acquire or to purchase. You will often see nabyć used in business contexts, such as when dealing with a dostawca (supplier).

What case is coś in this sentence?

It is in the Accusative case, acting as the direct object of the verb nabyć (to acquire). The pronoun coś (something) does not change its form between the Nominative (subject) and Accusative (object) cases, so it looks exactly the same here as it does in the dictionary.