After a while, you can create titanium with the Calciner building. It is unlocked when you research chemistry (60k science and 50 compendium, to give you an idea when it is going to be unlocked). Of course you can build more calciners or upgrades for the building (which I will not name, for spoilers). You can look them up in the wiki if you want to. Without upgrades it gives 0.0005 Titanium per tick, which is not much. It is however the only automated method to gain Titanium in the game right now.
There are two ways to get more titanium from Zebra's. This is the only tribe that will give you Titanium.
The first is to improve your standing with them. That is not done by killing their natural enemies, the lions, but rather build more tradeposts and unlock
Caravanserai from the workshop. For every tradepost you have, the standing is improved by 0.35%. The base standing is 70% for Zebra's. Note that there is the diminishing effect, explained here) . This percentage is the percentage of 'normal' trades for hostile races, such as the Zebra's. A random number between 0 and 100 is generated, and when that number is higher then the race standing, there is no trade at all (and hence no titanium). Higher standing results in more trades, more trades results in more titanium.
The other option, which is way more convenient I think, are having more tradeships. You need to have at least one to unlock the zebra's, so there is already a start. For every tradeship you have, the chance to get titanium from the trade is increased by 0.35%. The base chance is 15%. I cannot find the diminishing function, so at 243 trade ships there is a 100% chance to obtain titanium for the trade. Before you hit that number, the chance to get it is increased. Also, you get 2% more titanium for each tradeship you have! Double bonus!