> When I run the form, the grid will display all onhand items(with item name, received date to the inventory...) and I would like to create an array for all serial no.(the grid's first column) and store it under the combo box. If so, then I just click on the combo box to choose the item which has that particular serial no and go from there. But I don't know how to create an array like that.

Do you have to use an array? Why not just use the table/field source?