As an alternative to finding a large Yaupon holly, could you move one of the existing hollies from one of the ends to the gap? You could put some other shrub, or a smaller Yaupon, at the end, where it might be less obviously smaller or different....

I wish you luck in finding what you are looking for, but as a possible way around, this might work.

Is it Ilex vomitoria 'Nana', 'Shillings' ('Stokes Dwarf') or 'Taylors Rudolph'?

You might try John Deere Landscaping on Hwy 64 between Apex and Jordan Lake. They will sell retail once in awhile, but they'll ask you which holly you have. They sell the largest specimen shrubs and trees that I know of in the area. I just looked up the availability list of several of the wholesale nurseries that I use and they didn't have anything larger than what you've already found.

Cameron

did you call and check with broadwell's and/or old stage? they tend to have pretty big plants. good luck with your search!