OK, its now time to apply the piano wire springs.
I’ve looked in hardware store, without luck. Is this a music store item?
What note works best, middle C, or A-440, G- an octave below middle C, or some other note?
I don’t know?
I’d go with middle C. It’s kinda in the middle of things. 
I get mine at the local hobby shop in the K&S metal area, the shop has it because they do some model airplane stuff, so look at a shop that does that.
Yep,
Flying model airplane stuff
If your local hobby shop does not carry - look at some of the larger hardware stores like the big Ace stores - they will carry it. Also Hobby Lobby sometimes some of their stores have it (in my area, one does, another does not)
Steve F , if you want piano wire , I can steal some from the piano in the local community centre , nobody ever uses it except to annoy the crap out of everyone .
Failing that , use stiff string .
Why is it that people can’t walk past a piano without tinkling on the keys ? No , the other tinkle .
Musical Mike
Yup - Hobby store. I’ve tried every hardware store I can find with no luck. Most refereed my to music stores
Mike 
if you can find phosphor bronze wire, of a reasonable thickness, you can use that, too.