Looking for fat spokes.
 

I've got one of those Choppers US Super Fat frames, a wide S-A rear hub & 24x4 rear rim, and a single-disc & 24x3 front rim. I'm planning on lacing the wheels in 3-leading/3-trailling fashion: http://www.minortriad.com/wheel.html
My concern is that the wheels are going to look awful empty &/or flimsy with all that rim & hub & tire and proportionally skinny spokes tying it together. Is there a source for larger spokes and nipples? The longest motorcycle spokes seem to be for 21" wheels with even bigger hubs.

you can get bladed spokes, but it still would have a more fragile look than what you are most likely looking for.

Have you checked at local bicycle shops?

On the Clydesdale forum, people will periodically ask about heavy-duty wheels, and evidently the bike shops have available heavier spokes than standard.

Worksman bicycles use 11 ga spokes, not sure where to get them.

You may have to drill out the holes and may have problems with spokes being too crowded at the hub with larger spokes.

I couldn't tell from that picture, but are all the spokes the same length with that layout?

There's plenty of room on the hub to hog out the holes a skoosh; and, yes, the selected design has all spokes on the same side of the wheel the same length - 3x. The other side of the coin being that the front will be offset for the disc mount, and the measurements say the rear will be offset as well. So, once I get around to doing all the calculations, it looks like 4 sets of 18.

When I was living in Daytona Beach 35 years ago I built up a couple of wheels for my Atala track/city bike using heavy gauge tandem spokes on Mavic rims. Those boys were solid but the ride was kind of harsh even though the wheels were laced cross four on low flange hubs.