Check Classic-Enlargers, mentioned above, for lens disks. The flat board is for lenses up to 105mm, the next size (2-1/2" or something like that) is for 135 and thereabouts, and the long 4-1/2" cone is for a 150. You can also find the lens disks on eBay. Some disks have a flange attached, and some just have a 39mm hole, and you use a retaining ring that screws on from the back. You can get 39mm rings from B&H and most any dealer that sells enlarging lenses. Some older or long lenses may take other sizes.
Some lenses can mount in more than one way. My 150mm Apo-Componon comes with a mounting ring that works directly with the Omega 4-1/2" cone and no disk, but it is also threaded for a 55mm retaining ring, and you can remove the mounting ring, which has some built-in lens extension, and mount the lens in a lensboard with a 39mm hole, by removing the rear cell and re-attaching it with the lensboard in place.