Spokes in virtually all lengths are available though some lengths may need to be special ordered. A bike shop that does much wheel building should have a reasonably full range of sizes.

Current SON, SRAM, SA and Shimano dynamo hubs are similar in flange size to the old SA dynamo hubs I believe so spokes in the correct lengths should not be that hard to find. IIRC Peter White Cycles has a full range of spoke lengths available and builds wheels using the SON dynamo hub.

I have the Shimano Alfine dynamo hub on one bike and really like it. Amazing the light output with a modern LED headlight such as the B&M Lumotec IQ Cyo that I am using even at modest speeds.