My RADZ Hawaii adjustable harness lines have webbing and plastic buckles. Easy to adjust and havent had any issues with them. I am sure I have seen them in RPS.

Sailworks make some which are really easy to use. Snides thinks they are the best thing since sliced bread, or something like that.

Over here they are available for just over 60 bucks at Windforce. I guess they would post if you can't find them locally.

I use Vis'Spd' lines,

there good as I can replace the tube and rope that I wear out so often. With my weight hanging of them the buckle slips which is perfect for Sandy Pt. I start off with the lines a bit shorter and as I bear off down the course powered up the lines lengthen, the sail sheets out and my arms stretch out producing an ok result I guess.

Adjustable lines are great for fine tuning your setup and seeing what works best for you.