Where can I find a wheel of this design?

This type of wheel is fairly common on "tadpole" design recumbent tricycles, meaning that there are two front wheels and one rear wheel. Therefore, many companies sell such wheels in this size. Look for stores that sell replacement recumbent trike wheels.

The exact axle size is variable, so you should decide on the wheels that you are getting before you drill the hole in the frame to accept the axle, and then drill an appropriately sized hole.

A second consideration is brake mounting. The n55 instructions recommend mirrored drum brakes or disc brakes, but do not provide instructions for mounting disc brake calipers. You will need to work out how to do that. Using drum brakes will eliminate this issue, but wheels with drum brakes are much less common than those with disc brakes.

UPDATE: OP has indicated in comments that 1) brakes are not necessary and 2) cost is a concern. Given those constraints, some other options are

1. find a discarded wheelchair and take the wheels
2. DIY a one-sided wheel by acquiring two discarded BMX bikes with over-sized axles (20" wheels will be much easier to find than 24"). Remove the axles from the wheels and replace the front wheel axles with the longer ones from the rear wheels, adjusting the cone positions so that the excess axle length is entirely on one side. You may need to cut additional threading into the axle to accomplish this. If 24" size is extremely important, find a kids mountain bike with 24" wheels with the same spoke count as the BMX wheels and lace the BMX hubs to the 24" rims. You may need to get new spokes to do this. There are many nonprofit bicycle co-ops that have free or inexpensive BMX bikes for salvage to do this project.