As WhiteBears mentioned it's not in season yet. You'll have to wait usually near August. Although you may be able to buy some different cakes which has those fillings in Chinese bakeries or go to Wing Yip look in the cake section.
I hated moon cakes when I was a child, especially the brown sweet paste! I could just about tolerate the 'yolk' bit but only just! I've not eaten any since I was child and I'm sure I still would hate it now. I've seen them in Chinese cake shops or in tins at Chinese supermarkets but they are probably only on sale around the Chinese Moon Festival. I've never been tempted to buy any!