To purchase a creative world, use the command /world buy. You will then be prompted to purchase a new world for 10,000 coins. Accepting will grant you a 256×256 area to build in. You may purchase as many worlds as you can afford.
If you had a plot in one of our past plot worlds, you may locate it via /plot list. Once located, use the /plot warp # command to be teleported directly there. While you may still build in your old plots, new ones are no longer available, and thus purchasing worlds will be required for future projects.