Piano wire or broom wire. You can get broom wire from One-String Willie on his site.
I don't think that even upright strings will be long enuff.
Maybe call a piano company. IIRC, they get piano strings on spools ... cut to length.
You might be able to call around to hardware or fastener stores and find galvanized wire.
Here are a couple of alternatives, all of which are discussed in some detail on my website www.onestringwillie.com :
1) if you can find an old broom, you can get about 7 feet of wire off of it
2) I sell ~24 foot lengths of new broom wire on my website for $4 plus shipping
3) you can order 6 foot lengths of music wire (it may be spring wire) in a wide variety of thicknesses from www.smallparts.com
4) if you can find an old bicycle tire or an old automobile or truck tire, you can cut the wire out of the the two beads of the tire. The bicycle tire wire is somewhat thin, but still fine for a diddley bow, but the heavier automobile or truck tire is probably better for a 4-foot-plus instrument
5) I respectfully disagree with AFKAM on the wire from hardware stores--you need high tensile strength wire, and the stuff you get at the hardware store (at least the stuff I have gotten at the hardware store) will keep stretching and dropping in pitch as you play
6) I have not thoroughly explored calling piano companies as suggested by AFKAM, but this may be a promising suggestion--one thing to remember about music wire (and heavy tire wire) is that it is somewhat difficult to bend and cut.
Best regards, Willie
Try long neck Banjo strings.
Enjoy, Keni Lee