Rumors are that a certain space station always has a few nukes in storage. All you need is a few gold coins, a space center, a cargo shuttle, rent a dock at the station, and presto!!!
That being said, you will hemorrhage gold coins like crazy because it is highly expensive. Having done it myself, I'm not sure it was such a wise idea. In reality, when am I ever going to use nukes? If you use them, 4 or 5 innocent countries will be affected in addition to the country you attacked, and you'll have 2 or 3 angry new enemies looking for revenge.
The game designers have really sucked the fun out of using nukes. But then again, you can say the same about the war engine. I miss the days when wars were cheap and easy, and C3's were loaded with cash. A few cheap nukes and a few thousand drones, and victory was quick and painless!