If you can find a comparable cup with the seal, go for it. I've never seen the seals sold separately from the cups, but I may be wrong. Otherwise, you may have to buy both cups to get what you want.

BTW, the seal is not strictly necessary but acts to keep contamination out of the grease, rather than keeping grease in the cup. Since the fixed cup is ok, and that's the cup that determines chainline placement, you might be able to scrounge a replacement adjustable cup (with or without a seal) that will work with your spindle and shell width.

And replace the bearings anyway, as a matter of policy.