What do you mean by "cheated" did he "lose" them when ranking up because pretty much all of the rank ups cost seals. The final rank up will cost you 25k seals.

Another way to get seals is to do Chocobo Caravan. You will need to do the caravan in whichever nation your Grand Company is in until you can unlock "don't hate the messenger" quest, then you can do caravan in any nation for seals.

If you want to farm gear for seals here is a link that lists what mobs drop each piece and how many seals they are worth but be warned drop rates are pretty random and it may take you a lot longer than a caravan party