'course, with dust seal is better than without. But you don't really need it until you ride in dirty conditions.
I scraped by quite awhile by cutting a wide rubber band out of a discarded tube and fitting it over the top headset bearing.
Don't really know where to get a spare.
A completely new headset might be the easiest option.
Unless you manage to find a relentless tinkerer which might have saved the top half of an otherwise busted headset.