Leashes wear out just like line sets. You want to replace it every year or 6 months depending on how often you kite. I made my own leash from an eyebolt, wood block, carabiner, 5mm dyneema, but tend to use regular leashes more. If you have a friend who buys new kites with control bars and resells them every year, ask them for their old leashes, there are lots of old unused leashes out there you can grab for cheap.
If you have some broken leashes laying around, grab the eyebolt from them and you can make a new leash. To make it stretchy, get some elastic and draw it through the center (like how you splice) and then sew it on the ends.
The red one in the above looks pretty, thanks to the wood grain, but it's awkward to grab. I have another leash I made with fabric cover, easier to grab, anyway, making leashes is not too hard, I would go with the eyebolt + wood (or nylon) block method as it's cheap and works as well as buying one of those 2673 wichard shackles (they run about $70 for the shackle alone). You don't want a cheap shackle as they don't release reliably. The one in the above photo cost me $15 for the carabiner, $10 for the dyneema and I reused an old eyebolt from another leash where the splice failed so $25 total. Then I got another leash free from a friend of mine who buys new kite stuff every year so that's the one I'm using now it's a north, pretty nice, but they do wear out.