There are many sources of stock for stock loan. 25 years ago, the primary source was stock in the margin accounts of investors. Today, nearly every large holder of stock loans it out. The reason they loan it out is to make more money.
Virtually every broker/dealer (b/d) that holds their own accounts has a stock loan desk (small to medium sized firms frequently have their accounts an another firm's books on a fully-disclosed basis). Virtually all large index fund managers have a stock loan desk as well. The stock loan desk at a b/d will loan stock to the firm's customers from the available shares (more on that in a moment) or it will find another place to borrow the shares from on behalf of the customer. This can involve looking in a system called Loannet or merely calling up other participants in the market.
The original source of available shares was the margin accounts of customers. The amount of stock available for loan depends on the amount of funds loaned to the customers. The stock loan activity is completely invisible to the customer whose account the shares are taken. Don't want your shares loaned? Don't use a margin account. Stock loan is the financing mechanism that provides the funds loaned to you for your margin account.
Securities can also be loaned from fully-paid (non-margin) accounts of customers with the written consent of the customer. It's a pain in the ass from a regulatory and operational point of view. It's usually only done if the customer has a really large holding in a hard to borrow stock. The customer generally negotiates a share of the revenue from the transaction.
In the past 25 years, institutional investors have started to loan stock as well. The pioneers were index funds, but it has spread to most other fund types. The big institutional investors generally set up their own desk and participate in the market directly. Being a direct participant can improve their ability to borrow stock as well.
The borrowing party puts up collateral (100-110%) for the stock and the lending firm either uses the funds to finance the margin business or puts it in a limited class of interest bearing accounts (I forget the name and regulation) at a bank. The borrowing party gets the stock and promptly sells it. The amount of the collateral is trued up to the value of the borrowed shares on a regular basis, so the risk to the lender is small.
So it is clear that one reason to loan the shares is financing. The second reason is revenue.
The revenue comes from the interest earned on the collateral. The interest on the collateral belongs to the lender except for a negotiated "rebate". For most stock, the rebate is generally 10 to 25 basis points less than the overnight benchmark (fed funds). The lender keeps what they can earn over the rebate.
Notice that I said "for most stocks". Some stocks can be hard to borrow. The supply can be low because large amounts of the stock are held in non-margin accounts or by investors who don't loan it out. The demand can be high because there is a large amount of short interest in the stock already.
The negotiated rebate on hard to borrow shares can be negative, and not just a few basis points. The negative rebate for a really hard to borrow can be negative 10 percent and worse. And a negative rebate means that you're paying somebody interest to hold your money as collateral.
This can be very lucrative for index funds based on a broad index like the Russell. It's one reason index fund fees are so low.
At the other end of the scale is generally available stock. Known as GC (general collateral), loan transactions in this stock are usually initiated by a stock lender looking for financing.
From a b/d point of view, stock loan is one aspect of a business called prime brokerage. Prime brokerage is a bundle of custody, operational, financing, and loan services offered to hedge funds.
One note, the perspective I've provided is largely from the institutional trading side of the b/d business. Retail investors borrowing stock will generally see a tier of rebates (I think the most common rebate is zero).