I have at least once seen bigger hydrangeas in tubs at a local outlet. But this is not a regular thing. Due to the cost of shipping one big heavy plant in from outside the area I think you are stuck with whatever you may be able to find near your there somewhere.

A thought: given proper sunlight, water and fertilizer, a standard hydrangea shrub sold at a store -usually- will attain a large size in about 3 years. If starting from cuttings then, obviously, it will take longer but either will be cheaper than tying to buy a large specimen and pay large mailing charges.

Mail-order (ie, Internet) plant nurseries normally do not advertise these large plants in their websites so, I suggest calling them rather than doing a Google/online Search. You will be expected to come and pick them up personally.

I purchased Hydrangea macrophylla that large in a pot at a local nursery but these are not easy to come by. I would stick to checking all local nurseries as I cannot even imagine the cost of shipping a plant that large.

Thanks everyone, this is all really useful. I'll have a good look around local nurseries and try and find the biggest!

You might also want to ask realtors or new neighbors. We have new neighbors who want to clear out all the shrubs in their backyard and trees, too. So sad-they just want grass. Anyway, they have told us we may take anything we want! A realtor might be able to point you in the right direction. If it warms up after the snow tomorrow, I will be spending my spring break digging and replanting established shrubs!