It is sold by the roll. Red Brand is 100', 165' and 200' rolls. I guess they do make 72" height as well.
Horse fence is 12.5 gauge. They also make a kennel fence that is 16 gauge.
https://redbrandstore.com/collections/horse-fence
You should be able to find it in a farm store.
I built my house on a vacant lot in a established suburb. The houses around me were probably built in the 60's or earlier. The previous owner had put up a rabbit fence on some of the lot lines, He put a 6x6 in each corner, with diagonal 4x4s bracing them. Standard T-Posts between them. The fence was old when I bought the lot 17 or 18 years ago, and those corner posts are still fine. Parts of the fence mesh could use replacing, that's why I went with the heavier gauge on the section I put up. I don't want to do the job over again and I won't.
The neighbors had a invisible fence, their dog was out in the street all the time.