I missing pieces from a big bunk bedset how do I get the pieces I'm missing ??

If the item is new and you know the model name or can figure it out. You can call the store and explain to them & let them know what you are missing. If you purchased the item used, you can also call, but will require research of the model name and possibly what style or year. And most likely, you will have to purchase those items.

The item is a new bed i bought yesterday the wooden poles comes with the bed it's all in 1 big box I'm missing side pieces for the bed