Regular sheets have surface finishes which prevent dyes from taking. These finishes can't be removed. Even after many washings, they tend to dye splotchy. So I'm having them made for us without finishes and hemmed with cotton thread. That's taking a lot of effort so I hope you will buy large and copious quantities. Visualize everyone you know sleeping on tie-dyed pillowcases!
These do not have as many threads per inch as expensive, high quality pillowcases, but they work just fine. About 60 x 60 threads per inch. Made oversized, they will shrink to fit.
Cotton Pillowcases are currently on sale! Get them for 10% off, now through June 1st!