Find a lumber yard with a "cull" pile. They'll either give or sell you (cheap, cheap, cheap) their discarded, damaged, or otherwise unusable cedar fence pickets.
Use a saw (circular saw, table saw, hand saw, chop/miter saw) to trim off the damaged portions of the pickets. These are usually split ends, or large knot hole areas. Once they're gone, you'll be left with several sizable sections of cedar pickets.
Now, construct a "pallet". Determine the desired width of your pallet, cut some 2xs to that width, and, space them out as "feet" to support your planters. (Keep in mind that heavier planters benefit from closer spacing of the feet). Cut your usable picket pieces to the length you want your pallet to be, and nail or screw (I use outdoor Deck Mate screws) them perpendicular to the "feet."
Congratulations, you just made a "Planter Pallet!"
If you're really ambitious, you can construct a square pallet, then use a circular saw or jigsaw to transform the square into a circle or triangle. Just trace your pattern onto the pallet and then cut it out. A coating of water seal will help protect your pallet from the elements, and the cedar will actually help repel some insects. Or, just let it weather to match your cedar fence!
I constructed one of these to lay across some not-so-level ground for three raised planters. It has lasted approximately 2 years (without protection). It would've lasted much longer if I had properly spaced my "feet." The weight of the planters wasn't supported well where the "feet" were spaced too far apart.
All in all, I'll be making more of these for my patio planters. They look great, too!