Well you need trucks for having units, either ways...
You get trucks get looking at your top bar and one should say Trade, click it.
A drop-down should appear and then click Products in Stock: Buy and Sell
This will bring you to a large list of items you may purchase from the World Market, it is alphabetically ordered, so it should easy to find Trucks
Then once you've found Trucks go directly to its right on the other side of the page, you will see Buy/Sell, click Buy
Then type in how much you need, I recommend buying 10000 at 120 Q, the rest doesn't matter
Then click order, and don't click Immediately Order, then you'll pay more
Hope it helps