Try the usual big name shops- Raber's in San Jose, British Only
, etc. There are also several repops on E-bay, India made. But the biggest thing to be aware of is that there are two basic sizes- smaller ones for the lightweights, and larger ones for the bigger capacity machines. On a used seat expect to have to replace the 13 or so springs on the underside of the seat- usually they are stretched with one or two missing springs, this will cost about half of what you have just paid for the used seat that needs the springs! Search words include Solo, Lycett, and Terry.