Build One
I would definately build one because it will be much cheaper and you will have what you want and not just be settling for what you can buy. I hunt out of 2 box stands. One is about 25ft tall at the windows, and the other is about 18ft. The taller one was built by my father when I was very young but he built the legs and ladder out of steel rebar if im not mistaken. There are X braces between each leg to make it more stable for being so high. The room is just made from 2X4 and 2X6 with plexy glass windows cut into grooves so they can slide back and forth. The roof was done just as a normal roof is; tar and shingles. However, when we built our second all wood box blind we simply put on a tin roof.
Each blind cost around 1,200. But for being so high, roomy, and made just the way we like it, it was more than worth spending the money. The metal tower has a 6X6X7 room and the wooden tower has a 8X7X8.
This leaves room for heaters, chairs, and cots if you would ever like to do some extreme hunting and stay in the blind overnight as to not spook any deer coming in the next morning.