Those people make GREAT products. We just got a cinderella carriage from them and it is AMAZING.just looked at the price for a 5th wheel on that site,they,re not cheap,you may be better buying an old dray/cart with the turntable ,plus you might get your springs as well. what country are you in
As for the springs, look up trailer part suppliers or spring shops. The leaf springs on carriages are smaller versions of what are on trucks and trailers.