Do you mean the flats that hold the pots together? If that's what you mean, then the big box stores will usually give them away if you see them lying around empty. Just ask!

Or do you mean plastic pots? If that's what you mean, then just ask other gardeners. They usually have tons lying around!

Ask a landscaper.
And some convenience centers (our county's name for where we take our garbage to be compacted before going to the landfill) have recycling centers (informally). I've gotten good sized pots at ours.