Ores on Loka spawn differently than in regular Minecraft. Sure, you can go digging and grab some ores under your Town, but the real goodies are at places called Resource Isles. These dangerous yet rich islands can be found outside of each of the three continents. Let's take a peek at the World Map below to find one.
Here we've got Kalros' Resource Isle. Resource Isles (RIs) can be found easily on the World Map; they are off the main landmass and have a diamond icon on them. Depending on what you want to mine will dictate which RI you go to, as some have more of certain ores than others. Make sure to compare your coordinates to the World Map whilst sailing; you don't want to get lost at sea.
Resource Isles regenerate themselves over time, making their supplies limitless. They host huge caves as well! While certainly a miner's dream, RIs are not the safest place to roam. For one, PvP is enabled, and you won't keep your items upon death. Secondly, Illagers lurk around RIs - beware of the Evokers, Vindicators, and Vexes!
More information on Resource Isles can be found here.