Assuming you mean the bit with the screws in holding the spring down, I'd have thought your best bet was to knock something up from whatever scrap you have lying around - it doesn't look like a precision item.
No help to you I'm afraid, but your picture took me back about 60 years to when I worked in an upholsterer's shop on Saturday mornings.
One of my jobs was to put those zig-zag springs onto dining room chair seat inserts, and I lost count of how many of those clips I used.
I can still recall the taste of the tacks taken from a dusty bag and put into my mouth for use with a magnetic hammer when putting the hessian cover over the springs.
There were two workbenches in the shop, and we 'boys' used to fight for the more solid one that enabled nails to be put in with a couple of hammer blows rather than repeated whacks on the flimsier bench where things 'bounced'.
We also fought over stripping three-piece suites, prior to re-covering, so as to be able to claim any cash found inside.
I have made one from a strip of metal but it squeaks which is a bit off putting, also tried various types of tape and strapping but they dont last very long.
I did try the 2 local upholsterers I knew about but neither of them used them, or thats what they told me but I was told of another one who does this type of work today so will go have a chat with them.