The problem with stacking w/o stickers is mold and the resultant discoloration, but you won't have to worry about that with a rot resistant wood like red cedar as long as the weather is cold. The best wood for stickers is whatever you have as offcuts. If you have a tablesaw and planer, it's easy to make them. The wood should be at least air dry, but it's the mold thing again so you could use freshly cut stickers over the winter & replace them in early Spring. I made my first stickers out of exterior grade plywood that I had laying around. It's already uniform thickness so you just need to rip them to width. They're still good after 14 years. You may be able to find offcuts at a construction site. After a year or so, you will have plenty of air dried wood to make your stickers from. I try to keep my count at less than the national debt.
I like to make my stickers 3/4 x 1 so I can easily see the "good" face. If they're square, you need to plane both dimensions rather than just 1. I tried making some fancy ones with a groove cut in the faces to speed up drying, but some kind of bee loved to nest in the grooves, so now mine are plain flat sticks, usually tulip poplar or pine. Don't forget to paint the ends of your boards.
