I'm not sure you would have any luck at your local hardware store anyway as it is most likely a metric size setscrew. If I was in your situation I would be tempted to just retap the hole to an imperial size and fit a new setscrew to hold the line space lever in place. If no one else jumps in with the original setscrew dimensions over the next day or two I'll pull one out of my SMs and measure it for you on the weekend.