I purchased a seat from Northern Tool, Michigan Seat High Back Suspension Seat Model V-5300 @ $329. Even though the seat fit the operator station perfectly, the bolt pattern on the new seat was larger. I had a flat adapter bracket fabricated out of 3/16 flat bar with both the old smaller bolt pattern and the new larger pattern. Bolted adapter to frame, new seat to adapter and done. I had to fiddle around with the seat safety sensor but even that was really no trouble. Just use flat bar wide enough to accommodate both patterns, weld, paint and bolt right on. I used a local fabricator and the adapter bracket was reasonably inexpensive.
My review of the seat: It certainly isn't a lazy boy recliner but it is much better than the stock seat that came with my
L3700SU. Of course, sitting on a concrete slab would be a slight improvement over the OEM "cheap seats" but I notice the improvement every time I use the machine.