I think with a coaster brake hub, you're probably better off with the higher number of spokes.
The WTB Freedom Ryder 23 rim is available in 36 hole:
in 26 inch
http://www.ebikestop.com/freedom_ryd...rim-RM2243.php
in 700c
http://www.ebikestop.com/freedom_ryd...rim-RM2241.php
Here's a 40 hole rim but it's a 700c, the Sun Rhyno Lite:
http://www.niagaracycle.com/product_...ucts_id=415575
Here's a 40 hole 26" rim:
http://aebike.com/product/sun-cr-18-...m8489-qc30.htm
This place has some 40 hole rims if you scroll down:
http://www.tandemseast.com/parts/wheels.html