You can certainly replace the saddle for any standard rail type seat. One thing though - DON’T OVERTIGHTEN THE BOLT THAT HOLDS THE SADDLE CLAMP TIGHT AGAINST THE RAILS when you do. There have been some cases of people overdoing the tension on the bolt and breaking the plastic moulding that the seat is fastened to.
I have a fake strida and I replaced the rather mean saddle it came with. The ride is very upright and as you have found out, you put more weight on the seat than on a normal bike. I went for a pretty cheap unisex Avenir saddle which cost about £12.50. It is fat and padded and sprung too. It might look like an old womans bike seat, but my a*se is a lot more comfortable than it was before. I can easily ride the bike for twelve miles now just for fun, whereas before, it was nagging at me after three miles.
You can probably get them at your LBS, or from here if you want →
http://www.nationalcycles.co.uk/avenir-avenir-comfort-ergonomic-foam-saddle.html
The perspective in this photo makes the saddle look huge. It isn’t that big, but it is larger than the one it came with by a good bit.