If you can't find a haggis slow enough to shoot, try shooting some bagpipes instead. Trim the legs off the bag and stuff it with sausage meat and breadcrumbs. Noone will be able to tell the difference, I assure you.
What you need is a "Haggis Caller". Similar to a duck caller except when blown, it emits the sound of a female haggis on heat. Get ready with a net to catch the biggest and juiciest of the male haggi who will crowd towards you looking for their mate.
http://www.sipr.ac.uk/Whistlestop/haggis2.htm