Don't recall who ran that ad for singleshot wire springs, it was quite a while back.
Through the long years of production, Winchester replaced quite a few flat leaf trigger/sear springs with wire springs in many of their lever guns.

I've looked at doing that, I think I could improve the trigger slightly on a Winchester single shot by it.  Very similar to the ones I do for the rolling block and Hepburn but will have to have thinner wire.  I wouldn't make it double like a paperclip, though.  Too hard to make it consistent.  Single spring makes it easier to make a clean breaking sear.

I just haven't had the time to work it out and develop it correctly.  Making a good wire spring is slightly more difficult than it looks at first glance, to make it break cleanly without creep.

Picture of Rolling Block trigger spring made from piano wire.

That picture looks like one of my springs; I suspect it is.  I've made about 2500 to 3000 of them; maybe a few more.  They're not quite as simple as they look.  The shape is very deliberate; it's not accidental.

Making a wire spring isn't as intuitive and easy as it looks to make a top performer.  How the spring is bent and adjusted makes a huge difference in how it feels on the trigger pull.  For the same pull weight, I can make a spring that has more or less "creep" to it, by the method I bend it.  I do the end the way I do instead of the bend that Val does because it has a different feel to it.   

I was probably about 1000 springs into this before I started noticing that there was a considerable difference in how some of the springs felt in use compared to others of the same pull weight.  Being an engineer, I did a process analysis on my method of making them; and functionally found that I had 8 fundamental steps to the process.  I then value engineered the process to determine how to perform those 8 steps.  Then, I analyzed variances in those steps to identify which variances changed the final product.  From that, now I can pretty consistently produce a spring that has a good pull and minimal creep.   

Of course, essentially any spring that reduces 12 pounds off of the initial pull weight of the trigger makes such an immense difference that most people really don't care about that nuance.  You'll probably be happy with any wire spring that you do yourself.   

I can really make a much better trigger out of the ones that I personally install and tune on the action over those I just produce to sell for a generic use.  But, even those have improved a lot in final usability over those I made 10 years ago.