When my daughter went from the crib to the toddler bed we used the same mattress and bottom sheets. I needed a light cover for her because it got cool with the AC. I didn’t want to spend lots of money on a sheet and blanket set because I had plenty of crib sheets to fit the mattress and I knew that she wouldn’t be in that bed for a long time.
My solution was to buy a pretty flowered, flat, twin bed sheet from the thrift shop. I then cut it into two pieces and hemmed the edges I cut. Now I have 2 light sheets for a crib size mattress. They have come in handy for the next baby in the crib as well.