Leve quests and side quests drop some gear pieces and you can also buy some of the lower level gear for a fairly cheap price from NPCs. Some Notorious Monsters also drop gear and weapons, but you usually need some friends to help you kill them. At level 25 you can start doing Toto-Rak raid which drops level 30 gear and weapons. Moogle Box is a great site for browsing gear and weapon options. I recommend updating your weapon every 10 levels or so, and for arrows I recommend going Warped --> Iron --> Steel --> Cobalt. When you start doing Grand Company quests, you can buy steel arrows from them for gc seals.