If you don't know enough about demons to answer the first question, you likely shouldn't be trying to find spells to summon them.
Working with demons can be very gratifying, but it also isn't something you ever jump into without knowing as much as you can and studying as much as you can beforehand, that includes summoning, but most importantly, banishing.
I don't think anyone would give you a spell to summon a demon, especially since we don't know how knowledgeable or skilled you are which could [most likely will] result in you getting in trouble. Demons are really persuasive in getting what they want and if you don't know what specifically you're dealing with you can get into a lot of trouble. Research the specific demon you wish to summon.