According to the forum, it's possible to get Spell Tag from Misdreavus and Shuppet.

I catched over 30 Misdreavus and 10 Shuppet (Pokémon Tower Underground) and still not 1 item.

I'm doing something wrong?

Pokemon that hold these items only have a 5% chance to hold them. That's 1 in 20 encounters. When looking at the area-pokedex-tool (I suppose what it's called), you'll see that the catchable pokes that have the possibility of holding an item will have an icon n the bottom right corner of their photo.

 

If they don't have that icon, try hunting those pokes in another area. Also, to make life easier for you- Get a Sentret/Furret or Shuppet/Banette with the Frisk ability. Teach it Thief (on top of Celadon Dept Store) and you will never have to catch a poke to get its item again, and you'll be notified at the start of each battle if the poke is holding.

also, in some areas the pokemons u are hunting might not bring the items, see on the dex tab on the right of your screen if the pokes have a small yellow box in them.

 

gl finding them