Where to buy spokes?

double butted - thinner in the middle e.g 2.0 mm - 1.8mm -2.0 mm
trippl butted - thinner at the middle and and extra thick at the elbow e.g 2.3 mm - 1.8mm -2.0 mm
single butted - thicker at elbow thiner along the rest. e.g 2.3 mm - 2.0 mm - 2.0 mm

I used some triple butted DT Swiss Super comps for the disc side of some 29er wheels. They have longer thick section at the hub (elbow) to resist disc braking forces. They are also thinner at the threaded end than standard spokes (need 15g nipples)to reduce rotational weight. Work brilliantly paired wth some DT Revo's.

Super comp 2.0 / 1.7 / 1.8 mm
DT revolution 2.0 / 1.5 / 2.0 mm