how do you know? And how do you work with them?
there are conjurers out there who will summon and bind them and other types of spirits, including demons. the costs vary depending on what beings you seek to work with. creepy hollows has a forum and shop where you can look at the list of spirits they can contact. other reputable conjurers include lisa from mystic morning treasures, billy from the pagan shop, jules from customs by j moon, rebecca from starry1night, lilith from lilith's treasure tavern, priestess eve from eden's apple, and melissa from jinnandtonics.
my advice is DO NOT work with demons before you have any experience in spirit work. they're black arts and they can cause lots of issues if you are inexperienced. don't be tempted by the thought of them tormenting people you hate - they demand a lot and will turn against you if you are inexperienced with black magick. good luck!