Some company, maybe Micro Art Studio?, may have done resin ones years ago, although those are probably no longer in production nowadays. Some sets have quite a few spares however, so it may be worth asking on the trade page? I think the Victrix peltast set for example has both enough helmeted and unarmoured heads for all figures (or near enough), some of which I'm pretty sure of Corinthian design. How many do you need?