if you can, bring them the old spoke (both pieces)
edit: if you can't, and want to figure it out for yourself, you can calculate the length using
spocalc. you will either need to measure the rim and hub yourself, or obtain the dimensions (effective rim diameter, center to flange distance, flange diameter, and spoke hole diameter) from the manufacturer.
for burly wheels check out tandem rims, high spoke count, 3x. but none of that's any good without a good wheel build. i'd ask around for who's the best.