When looking for low-cost pallets, seek out businesses that use pallets and negotiate with them, watch for pallets outside of businesses, and consult pallet recyclers about the type of pallet you want. Networking is the key to finding used pallets for your projects.
Especially if you routinely use pallets, build a relationship with companies that use them. When they are done with the pallets, they may give them away or sell them for a nominal fee. Be aware, however, that they may have to return them. If so, ask them where the pallets originated, and try to negotiate with those companies.
When you find pallets outside a business, ask if they are being discarded. Do not assume you can simply take them.
Recyclers often have a variety of pallets otherwise destined for dismantle. These recyclers can help you locate specific types and supply you in the future when those types are available.