Restrict Order Line Items to Supplier Part #s

np,

this is easy to do. It's called a conditional drop-down. Sometimes abbreviated as CDD.  

First, be sure that the order lines table has a lookup field for Supplier.  I gather this is a purchase order to a supplier so that the purchase order itself only has one supplier. 

Gi to the relationship where one item has many order lines and go to the field for related item at the top right side of the relationship. Edit the field properties and select Conditional. Then configure the conditional settling, to say that when supplier is selected, only show items which match  in the supplier field of the item.