David,
Many model shops sell straight lengths of wire under the guise of 'Piano Wire'. It is usually in a supplier based display – a series of tubes containing wires of varying diameters. This wire is not traditional piano wire, it is somewhat softer and less springy – I believe it is actually 'Music Wire' and has quite different properties – much more easily bent for instance and it will fracture on a tight bend. It also appears to come in imperial or metric diameters as opposed to the usual SWG.
True SWG piano wire is getting hard to source but the last place I found it in good quantities and full range of diameters was a SLEC at Watton in Norfolk. A quick Google will get you there.
Hope this helps – I can certainly sympathise with your situation having tried in the past to use this poor substitute myself
Regards – Ramon