There's always ebay if you can't tune it...

Or you could play it as a wide necked 6 string.

It's not hard though to tune your ears to hear an octave (and good training). Play the one note and get the other close with your tuner. If you twiddle the tuner up and down you should soon hear when they are the same note an octave apart - once your ear gets used to it you are on your way. If they are that close that you can't hear they are wrong just move one string away and it will clash like mad - it's then easy to take it back until they sing together.

One other way is to tune it in front of a TV screen - which acts a bit like a stroboscope. No I'm serious. If you pluck the lower note and move the tuning thing the other note will start to vibrate in sympathy when they are in tune. Not always practical in a pub or performance place unless they are kind enough to leave the telly on :)