Try going on GTS (Global Trade Station). You can search a Pokemon what you want and trade one of your Pokemon in your boxes.

Just beware: Most people on GTS are not being very honest, mostly because U.S.A. hacking their Pokemon, making and unfair or impossible deal. Most of us are very tired of seeing kids hacking their game, so look for a Pokemon you seek and check if it's a fair deal to you.

You may also offer your Pokemon and tell people what you want. After that, you got to wait. Patience.

Beware: If you haven't checked GTS for a long time, you may or may not get your Pokemon for your offer. And do not make ridiculous deals otherwise you will NOT get you want. As well for other low-life offerers.

And just a quick note, you have to make your deal in time before someone makes the deal first and it will be too late make a deal now.