If you are looking for small quantities of Ditra for tiling purposes, call your local flooring stores and someone should have stock rolls they use or left over plunder they would be glad to sell you. You may need to call several stores to find one that has what you need. This will save you the cost of purchasing a full roll for such a small project.
Another method is to use 1/4” hardi backer and thin set that down and nail it if you are on a wood subfloor. These sheets come in small quantities and can be purchased at the same stores you would call above or tile suppliers including box stores.
Good luck on your project.