You can just go to IKEA and ask for the missing pieces. Taking a sample, or the manual with the image of the piece you want would help.
I have a box full of extra IKEA pieces. If you tell me what it is, I could check it out
I bought table and chairs in IKEA. Chairs had ruined screws (melted metal made them impossible to... screw) and 2 pieces where missing from table. Didn't have to pay extra for them. Just smiiiiiiiiile

)
P.S.: The spare part counter is next to the client help in IKEA Dietlikon, just after the IKEA Food/ Hotdogs, outside the shopping area itself. Should follow similar plan in other filials.