Well, first of all, you have to make sure you're grinding on mobs that drop SoX possible items. Only the first tier of each degree can be SoS/M/Sun -- for example: weapons, the only SoX levels are 1, 8, 16, 24, 32, 42, 52, 64.
I have a level 32 Chinese character, which I farmed at Demon Horses (drops level 24 weapons) and never found anything SoX. How rare an SoX is depends on the level of Seal.
Seal of Star is least rare
Seal of Moon is more rare.
Seal of Sun is most rare. Some say you can go years on the game and never find a Sun drop, which sounds accurate.
I just started playing my European char and in one day I found 2 SoS drops. I believe it's all based on luck. I'm also rather sure that it is on a loop, meaning it drops a certain amount of SoX items within a specified amount of drops in that area. An example would be: 1 in every 1000 drops will be SoS, 1 in every 5000 drops will be SoM, 1 in every 10000 drops will be SoSun. This is not just for your character, this is for drops that everyone gets. So for each area I think it's something like that, but those are almost certainly not the correct numbers, just an example.
Hope This Helps,
MrMoney