If you're building an AD yi, you will have mana problems if you want to be able to cast everything over a longer period of time. You'll basically burn all your mana on 3-4 alpha's so there isn't much you can do. If you go AP yi, you'll have it easier mana wise since some AP items have mana regen, but thats it. Get blue if you can and just learn to not use spells unless you're attacking an important target (ie. Baron, Dragon, Red/Blue Buff, Champions).
For you build, I'm assuming you're laning, so I've heard from people that stacking Doran's Rings helps even IF you plan to build AD because it gives you the mana you need to spam alpha and sustainability through meditate. When I play Yi, I jungle. I go cloth armor 5x hp pots and will end up with items Wriggles, Merc Treads, and Ghostblade. Then I'll finish with FoN, Atmas, and Warmogs.
Oh and manamune would solve the mana problems you have, BUT its a wasted slot because your current yi build has 0 survivability that one stun and you're dead from burst.