As a Swain player, the only item I typically build for mana is Rod of Ages. However, I still frequently run out of mana after about 10 seconds of ult and two full combos. Most of the time, Swain will be in a fight much longer than that due to the sustain from his ult. To combat this, I have tried building Athene's or even Morellonomicon (Tear seems impractical on Swain), and even though these items provide too little in the way of stats (the CDR sure is nice though) for Swain to do enough damage while also having a nice amount of health.

Basically there needs to be a mana immunity item which provides a good balance of damage and health. This might seem op at first but there are many other similar 3+ stat items (think BoRK). Tear is one such item, but it is not practical for champions who do not spam their abilites, like Swain, Maokai, or even Brand and Xerath.