Can't help with the question of where they get it, but you can get some line if you want to do this yourself.
Marlow "Excell Pro" and New England ropes both make some small cored line you can take the core out of. I used to do this for sleeving on my Q-Powerline line sets I used to make. You can get these 2 brands at most sailing/marine stores.
But now, with Jerry Brown linesets I make, I just use Bass Pro Shops Offshore Angler "Magibraid" hollow core Dacron line in 200lb test. You need to seal the end so sand does not get under it and work it's way down to the larks head loop. If it does, it will fail prematurely there from that sand infiltration.
Another hint is to wet the line Q-Powerline before you machine sew it. This will help the thread to slide past and not overheat (thus cutting) the tightly bound internal Q-Powerline strands. For braids like Jerry Brown or regular kiteline you splice, I recommend hand sewing it to lock the splice in place - thus you do not need to worry about wetting it. And definitely go with a splice if the line braid will allow it. Splicing is way better, though much more of a hassle than sleeving and sewing.
Includes everything you need, except a sewing machine.
Exactly. go to that link and spend $8 for just the sleeves and get about 2-3 feet of cord. For $8 you can buy about 100 feet of cored line with the core in, and all you have to do is go through the bother of taking the core out. I can't remember what I bought last time I got a roll of this, probably 325 tactical paracord.
Matteo, with sufficient practice and the right tools / line types stitched splices can be made faster than a stitched sleeve.
I guess I would agree. And I am prepared to take some flak for this, but I spend about 2 hours making a front line set - from making the sleeving and inserting the line, to end splice, to stretching, to truing up for the opposite side splice, to making the opposite side splice, to whiping the sleeving end, then install on the bar.
Stretching setup, actually stretching, and take down takes the most time in this operation as I do multiple stretches on each line in an even sequence.