Sulfur is usually used to aid with banishings and/or hexbreaking. Since I don't know the specifics of your spell it's hard to say what would be the best substitute. Most likely it would be other items that aid in banishings like basil, clove, dragon's blood, garlic, cayenne and black pepper.
I'm making Dark Art's oil and am looking for a replacement for Sulfur as it's hard to obtain where I live. The recipe I am using is the one from the Black Witch coven which calls for Grave yard dirt, Sulfur, Agrimony, Black Pepper, Spanish Moss,
Patchouli, Pine, Peppermint, Black Dog hair, Mullein and Vervain. What do you suggest would be a good replacement? I have seen other banishing oils that contain sea salt, cayenne pepper and castor oil as a carrier oil I guess.
Personally I use tobacco when I don't have any sulphur and find it works well. It all depends on how you feel with it, some people may say to remove it completely if you don't have a replacement or find something you feel fits your requirements.
Sulphur is also often available at gardening shops, as well as online so if you really want to use sulphur, look for it there too.
Match heads contain some sulfur. I've known people who would light a match to 'activate' the sulfur, but carefully crushing off a match's head (be extra carful in general; avoid it altogether with strike-anywhere matches) can provide a small amount for spellwork.