Availability depends on where you are located. Which is something you have not stated.

Yes, with enough money you can buy them, but it's very unlikely that a mature root system will come with them, and they may not do well long term due to that.

Up to you.

I've read many times that while planting larger trees may give you greater impact initially, you just can't get enough root mass to support them. They may survive, but the transplant shock will be much greater and they won't thrive for many years. Save your money by planting smaller (younger) trees, and in a few years they'll be as big or bigger than the much more expensive "mature" ones would be in the same amount of time.

Given the logistics of getting them to you it's not something an online-only vendor would really be a good fit for. You're better off talking to a local nursery. There's usually a size where a plant goes from kind of pricey to holy crap, and someone knowledgeable in your area can help you figure out if it's worth it.

As far as viability, it depends on the quality of the vendor, the health of the plant, the time of year, and how it's handled. We did a bunch of 10 ft Norway spruces for a client last year. The rootballs were massive and required a tracked machine to offload and place them. They're pretty happy. We designed a landscape back in 2012 where they brought in 25 ft cryptomerias and they're all doing well. Some plants do well, and some are more prone to transplant shock than others (looking at you, Foster holly). It's why working with a pro who knows your local area makes so much more sense.

location ... big city name.. as well as a pic or 2... will probably get you your best advice ...

i ran across this place surfing the web .... https://www.instanthedge.com/ ..i had good dealings with a sister business buying mail order conifers and Japanese maples ...

understand.. the larger the transplant.. the longer you must provide followup care ... to insure that your investment doesnt die .. usually that means proper deep watering.. and then near drying .... until the plant grows the roots.. to support the large plant above ground ... though we can offer suggestions on how to do such watering .. it is mostly left to you figuring it out in your yard.. in your soil.. in your location .....

and keep in mind.. when you plant a big hedge.. sooner or later.. you will have a big annual or twice a yer job of shearing them.. to maintain them ... nothing really stops growing at some magical height ... so it will be up to you.. or your service to keep them in the desired shape ..including height and width ...

i dont grow boxwood ... but i am aware of regional problems with such.. im not sure if that would be a wise choice ...

ken

Not many trees are "full grown" at 10' :-) Even a great many shrubs will exceed that height when full grown so a very mature or full grown plant may not be what you really need. If you can better describe your need for privacy screening (how much planting room available, how tall the screening plant actually needs to be to do the job) and provide photos of the area and tell us where you are, we might be able to provide some suggestions that could work.

American arborvitae, Thuja occidentalis, is often used for privacy screening in many parts of the country because it is evergreen, dense, very narrow and grows just about anywhere. Here, the most popular cultivar, 'Smaragd' (aka Emerald or Emerald Green), is readily available in larger sizes....8-10'. But there are other plant possibilities that could work as well but we need to know your location and specifics of the situation to make the best suggestions.

As noted above, there are inherent issues involved when planting mature trees: the costs involved - not just with the trees themselves but also with transport and installation - delayed establishment and increased aftercare.

Last year I purchased eight 8'-9' emerald green arborvitae for $69 each from Home Depot. Had them planted for around $400 I think.

I think the big ones sell out fast and I was one of the first customers to pick over their selection, picking the best ones I could find from 2 stores.

These were a relatively cheap, instant solution for me.

One can buy any size tree they can afford.

I do not know of any nationwide internet sites, I have used local landscapers who have their sources.

Sometimes, one is unable to source vegetation from a far distance because of diseases and pests.

I recommend contacting local landscapers and nurseries.

Are you looking for suggestions on the plants themselves or where to go?

Larger transplants will greatly depend on the species. For example a smaller container oak will likely catch up to a balled and burlapped oak but if we’re talking maple I’ve never had the smaller container maple catch up to a balled and burlap maple.

What you want is a tree spade service who can source trees native to your area.