I have two questions:
1. Where can I find large limelights? I cannot find any that are bigger than 5 gallon pots and taller than 2-3 feet.

2. How close do I need to plant them to achieve the density that you see on the photo? I want to plant then this fall and I want the shrubs to touch next spring as they come back to life, I don't want to wait for years for them to touch. The instructions say they should be planted 6 feet apart, but that will leave me with huge gaps for years!

Thank you in advance.

I would contact several landscaping companies within a maximum radius of 2-hrs (driving time). Two hours would be the most I would tolerate driving if I ever have to go to their offices so adjust for what feels adequate to you. Have them search for you.

You can also contact nearby hydrangea nurseries. Nantucket Hydrangea is in the northeast and probably the closest one I can think of at this point. Hydrangeas Plus is another possibility; they are on the opposite coast, though, in Oregon.... but they sell 3-year old specimens for $50.

Another thing to consider is that hydrangeas grow fast so, while it may be hard to locate large shrubs like those on the picture, you can grow them from smaller shrubs.

To get instant satisfaction, you may have to find someone who has a large row already and is willing to sell the shrubs. Transplant them into your planned site carefully so they do not dry out and place them 3-4 feet apart.

It is a very hard lesson to learn or trial to endure for many homeowners and/or novice gardeners, but patience truly is a virtue when it comes to gardening or landscaping. In this case, instant gratification with a full and lush hedge after only a few months in the ground is not going to serve you well long run.

If you must, Luis' advice about contacting landscapers in your area is good. They have access to the wholesalers who are likely to have large, mature plants available. You won't find them retail and certainly not online. Be prepared to pay for them - plant prices escalate geometrically with age, size and maturity. But do not crowd the spacing - allow the full 6' spread between plants and live with the "gaps" temporarily. Your hedge and its ultimate health and appearance will be far better for it.