I ordered a Hornady 9mm pistol die. It came in stamped 9mm/380, but there wasn't a spacer in the box to change from 1 to the other. I know when I got and RCBS 357/38 die there was a spacer included. Does Hornady not provide a spacer or did it just get left out of my box?

I'm not sure you'll have good results trying to size both on the same die. The 9mm is slightly tapered case & the 380 is straight.

You didn't say which kind of die it is. A full length sizing die won't work for both, as above.