The Soul Is Not Required Collateral
(But it Provides Good Leverage for the Fiend)
In the MM, page 51, a demon is described as
regarding any mortals in its service as tools to use and then discard
at its whim, consigning their mortal souls to the Abyss.
As written in the Player's Handbook, the Warlock's pact does not necessarily involve the Warlock's soul for the transaction, or the relationship, that leads to powers being bestowed by the Fiend as patron. The discussion on PHB 108 and 109 declares that the patrons
expect significant favors in return.
Furthermore, the Fiend desires
the corruption or destruction of all things, ultimately including you {the warlock}
Nowhere is the bargaining described explicitly as the soul being traded for the powers. There are plenty of other ways to destroy or corrupt a Warlock. Consigning the soul to the Abyss, or making the Warlock wear a metal bikini while chained to the Demon for eternity is one way it could play out, but it isn't a necessary condition.
Were one to try and raise a Warlock whose soul was the collateral, the Fiend
May not wish to release that soul,
May consign the soul to the Abyss and then no longer cares
- May require some form of payment or other favor.
On point 3 ...
- "Bring me the heart of a virtuous priestess of (x deity)"
- "Bring me an innocent child of Prince (____)"
On point 2 ...
- An adventure to the lower planes! Mission statement: recover the soul of our party member, the Warlock.
The options are only limited by the imagination of the DM. So, work with your DM.
If you are the DM, figure out a bargain, up to and including the soul as collateral for power, and then apply leverage. That's what demons do. (The trope is as long standing as the idea of making a deal with the devil/demon for earthly power. )
For example, a Warlock whose contract is with a Fiend, per page 105, may lead a cult. In that case, the Fiend may demand/require that the Warlock feed the Fiend the souls of cult members, offering them up in some exotic or dark ritual. The Warlock's soul is not at risk so long as the quarterly payments are made. Miss a payment and you get a collection agency showing up at first, a few lower level demons, and then it gets more interesting.
As to "are there any on-line" sub question, you can take a look at reddit or a pact example like this. The favored beer in Hell is apparently Falstaff. (Taken from Marlowe's Play Dr Faustus, wherein Falstaff and Mephistopheles work a deal ...)