Do you have a B&M bargains near you? Saw lots of Marvel stuff in there a few months ago when I was looking for Spider-Man stuff for a birthday present (notebooks, pens, boxers, money tins etc). Whether they'll still have them I can't say but worth a look (or quality save & home bargains tend to have similar stuff)
Smyths have the sets of super heroes and super villains, they’re the imaginext ones. They’re £15.99 so maybe on the expensive side if (like us) you do stockings for little inexpensive bits and bobs. You could split them up though and wrap them individually?
The range has loads of marvel stuff for stockings. I've done a superhero themed one as well and got tons from there. I've got blind bags, top trumps, a torch, bath stuff, pez dispenser And lego figures. Plus some chocolate (not superhero themed)