A) Build them buy building nuclear missile corporations, nuclear missile battery corporations, and a strategic base corporation in addition to at least two plutonium corporations per nuclear missile corporation since you cannot buy plutonium on the world market either.
B) Wait until you are no longer a new comer to the game and don't have to ask 'how can i buy nuclear weapons?' and by then several veterans that you may have befriended by then may be willing to sell you some via direct trade.
C) Find an equally new comer to the game that has already began building nukes and it is likely he/she may sell them to you without any qualms.
D) Join a successful and large federation where you will have the opportunity to be trained and taught of all aspects of nuclear weapons and why a veteran like Jack Frost simply responded to your question - 'you dont need to worry about that' and once your fed mates deem you knowledgable and disciplined enough they may be willing to sell you some nuclear missiles.
Brief Nuke Lesson: You can't use nuclear missiles by themselves, you need nuclear missile batteries and a strategic military base to successfully launch a nuke. Nukes cannot be used against C3s. Nukes cause fall out and damage multiple innocent countries which becomes expensive paying restitution for and causes new players and veterans alike to become angry/hostile toward you. You must come along way in the game before worrying about the nuke scene.
If you need a mentor/teacher/game friend I will be happy to take you in and school you on this game. Message me in game at the country p Priest Empire. Sincerely, Tattooed Priest