Dtel would be a good source. Leland cypresses as you say grow very fast. If you took one out as necessary, depending upon original spacing, the others would grow to fill the gap pretty quickly. I had a landscaper draw up a plan for me once, he wanted to put in the lelands as a barrier all around the back and up half the sides of the property. I used nellie r. stevens hollies instead. Haven't lost one yet, they are one tough shrub that get up to 20 feet high.