Where to buy steering box seal?

[blown motor]

I need a new seal for the top end of the power steering box were the steering column shaft goes in. Where do I buy that, having trouble finding one? This is for a truck with a 78 D100 cab on an 83 W150 chassis. Would the box go with the cab or the chassis?

[mopar jack]

Rock auto should have either year. https://www.rockauto.com/en/moreinfo.php?pk=846&cc=1084564&pt=7353&jsn=541

[YYZ]

Your local UAP/NAPA may also carry the seals, as well as Canadian Tire.

Does the W150 use the Saginaw steering box as well?