Backorder tracking

Backorder tracking

We are a wholesale company import from different countries. We get shipped by ocean containers. It takes up to 3 to 4 months for shipment arrival.
At least half  of the items we order is non inventory items because it’s custom made for particular customer. 
Pl let me know how to order non inventory items that will not have sku and the ways to track those items once it’s ordered. Need to know when it’s going to come