order form and order details

I have built and order form and placed a sub form with order items. What I need to do is when the user selects an item to add to the order, it needs to look up the cheapest vendor of that item from another table and populate the vendor, price and item description to the order item record.

Any suggestions on how to tackle this? I am new to Creator and Deluge script but not new to programming so technical speak is fine :)

Thanks