I found this shop located at Petaling Jaya that sells a large array of Halloween decorations such as scary masks, toy pumpkins, skeletons etc.
If you stay in Petaling Jaya area you can check out this shoo located at PJ Old Town. It opens till evening on Saturdays as well but close om Sunday and public holidays.
The address and contact details is as follows:
Hewsonix Store SB
No 95, Jalan 1/12, 46000 Petaling Jaya, Selangor
Note: You may also try to search out Halloween items online.