Especially in the early stages of plantar fasciitis, silicon heel cups may be helpful. For a lot of people just using these has helped them to get rid of their plantar fasciitis. For more severe cases, it may be better to use inlay soles.
Heelcups can be bought at many locations in shops, groceries, or on-line. Usually they are not very expensive, mostly under $25,-. The more expensive types are usually made of a more durable kind of silicons.
The main reason that they work well is the raised heel, combined with the cushioning effect. The raised heel ensures less tension on the plantar fascia, which is very beneficial. The cushioning effect under the foot makes them quite comfortable to wear as well. Sometimes they have a little cutout under the heel to further releive the heel spur location, but opinions do not agree whether this actually works.
Also pay attention to durability. Cheap cups often loose their flexibility, losing the cushioning effect.