Google searching should find a US agent. Here is a UK review site that you might find interesting anyway:
http://www.atob.org.uk/Electric_Buyers%27_Guide.html#Heinzmann
They say of the Heinzmann:
"The Heinzmann motors are made in Germany and have a deserved reputation as the best hub motors around - they're relatively quiet and reliable, but starting to look a bit dated, although changes are in the wings). Range is less than average, but with a charge time of less than two hours, you'll be ready to do it all over again after a leisurely pub lunch. Or at least, the bike will. The motors are also available built-up into a bike - the Heinzmann Estelle. Typically German, the Estelle is a competent and well-equipped machine.
Most Heinzmann bikes use a rack-mounted NiMH battery, but the kits are also available in the UK with cheaper, heavier (and slower charging) lead-acid batteries as the E-Go.
We have a number of road-tests of the Heinzmann "