As you can see from the previous post the correct stuff is called bonded polyester. Dabond is a brand name and the most common.
The V number is the thickness. I have used V46 for all repairs because that is what I had. You don't really want to use thinner thread for sail repairs because the thin thread can tear through the stitch holes (according to my friend the paraglider and kite maker).
You can buy it from Sailrite but those guys charge US$50 for postage outside the US. I get mine from a sail maker and you can usually get the end of a bobbin for a few bucks.
If you can't get bonded polyester then normal polyester thread is ok but has no UV protection. Upholstery thread is also good if you need thick stuff. You can get it from any sewing shop. Metler Metrosene Plus is ok and Gutermann is good.
Don't use nylon or fishing line. It's too slippery and the stitches bunch up.
I have used dental floss to hand sew a repair while on holiday (on an island in Fiji). I would not use it for a proper repair. It is not really all that strong and I doubt if it would go through a sewing machine.
BTW. I just bought my girlfriend a really good sewing machine for her birthday. I'm almost tempted to ding my kite so I can try it out.