For Christmas or the usual craft shopping, there are 3 stores that you can go and take a look when you are Petaling Street. That is after you have tried searching for the decos at hypermarkets and if none catches your fancy.
These stores are located on the same road around Petaling Street or KL Chinatown area:
Living Cabin(check out the cute bear head design):
Macy Handicraft at Petaling Street:
Nam Thong Gifts and Decors:
Macy and Nam Thong would close after about 6pm or 7pm. If you are there in the evening, you can also find lots of delicious food sold outside Nam Thong.
If you want one of the kind items, you may need to get them from here but if you are just looking for generic Christmas decos, the ones sold at hypermarkets are more reasonably priced.