What is the fastest way to get "fresh" united miles into my account without flying?

I want to take advantage of this United offer ASAP, but I don't have miles in my account.

I've thought about transfering miles from US Airways, but its a terrible exchange rate.

I've thought about purchasing something through the united shopping link with a registered card. How long does it take for the miles to show up in the account?

Does anyone know of a faster way or promo?