My experience with trailer springs that are available here in the North America is they are of very low quality.
I don't know what the maximum weight that the trailer will end up being, but people of the Portal have used Suzuki Samurai springs on trailers, others have used Jeep springs.
People have even used stock springs and de-rated them by removing leafs.
I'd start looking at what ARB is offering for vehicle springs, find something with a military wrapped eye with in your weight range, and the approximate length you need.
I don't have specific recommendations was we don't use leaf springs.