Hi,
I have been told by my fellow dutch worldcyclists that 40 and 48 rims are not sold commercially. You'll have to look for a tandembuilder, they have these rims custum-made for them.
Exal (http://85.10.200.53/exal.be/) sells rims for 40 spokes. They are not depicted on their website but they produce them. Just write an email
Mavic also had a 40 spoke rim but I do not know if it is still being sold.
But why do you need such high numbers of spokes? 36 spokes should be enough to carry the weight of you, the bike and luggage (upto 30kg).
And if you are traveling on bad roads MTB rims would be better...
Cycle the world (2 MTB rims, 32 and 36 spokes),
Marc