This worksheet is called 'Number Words to Spell' and it is also available for free at http://www.lauracandler.com/. This compact one-pager conveniently lists all numerals 1-20, 30-100, as well as 1000, 1000000 and 1000000000. The second half of the worksheet duplicates the first, allowing you to effectively use one sheet of paper for two students, if you just fold it and cut in two pieces.
Use this worksheet in all kind of activities with numerals, or just give it out to your students for them to always have it as a reference sheet for the correct spelling.