You can buy them individually from Olaf for 100k per coin in Lith Harbor. No, Sodane has daily quests you can do over again that give you a specific amount depending on what boss monster you kill. You also of course get them from doing the mainstream quests received from Wence.
You need to complete the quest chain (gaining coins as you go) before you can do the repeatable quests for more coins. As mentioned above, the quest starts with the Wence NPC in Perion (the NPCs in different locations aren't the same). Afterwards you can get repeatable kill quests from Sodane to get more coins (once per day).