People looking for donations for a small organization can consider contacting organizations like Computers with Causes; people working with schools and larger organizations might be able to solicit donations in a newsletter. Another option is to run a fundraiser and purchase discounted laptops.
Laptops are relatively expensive when they're new, but they lose their value quickly. As a result, people and organizations often seek to donate laptops instead of spending time trying to sell old devices, which typically yields a relatively small return. Running a small ad in a local publication can help organizations find people willing to donate, and posting fliers at local businesses can lead to donations as well.
People who need to furnish a fairly large lab might want to consider asking medium and large businesses, which often purchase new computers on a regular basis. Some businesses might be willing to donate computers in exchange for a note of sponsorship or other public acknowledgement of their donation.
It might also be worth asking for damaged computers and performing repair work. A failed hard drive, for example, might cause someone to donate his computer even though replacing a hard drive is fairly simple. However, this strategy often works better on desktops than on laptops since desktops typically use parts that are easy to change, while laptops often use proprietary components.