If you look under banner on ebay you will find several companies that sell them. cost wise, even with my labor, it is easily 1/4 or less the cost of vinyl wrap installed.
I may have let the company I use slip at some point, but that is one of those consulting secrets now. but I am pretty sure I talked about this on lawnsite somewhere.
I do go with a heavier weight banner, 20 ounce material as it will be on the trucks and not just hanging on a building.
I get pockets sewn in on all 4 sides, and how I mount to truck it is kind of a secret as well, but I will give you a hint, you need to be able to tighten it in 2 directions. the tighter the banner is installed without wrinkles, the better it looks.
I have my local marketing company that designs my website and postcards draw it up in vector format at 300dpi, when you do a banner 84x180 for a boxvan, you need a very clean non-pixelized print. you really need a great photo from a 12-18 megapixel camera with no noise, and even then my marketing folks interpolate the image to get rid of pixelization.
I have one on a boxvan that is 3 years old that has probably 2 years left on it, and changing it out takes 2 guys about 5 hours once you have the hardware on the van. The first time took 2 guys 2 days. but we got quicker each one we did.