Parts for most Husqvarna and STIHL forestry helmets are available from full service dealers (shops, not big box retailers). Each has a few different models, as well as changes through the years, so bring yours with you.
I have replaced suspensions and face shields. Ear muffs are the most expensive components, and at least one dealer advised me to just replace the entire helmet. If you buy the same model you may be able to reuse some parts.
The MSA helmets are sold through safety supply vendors, and places like Grainger and Fastenal. They offer a wide range of choices.
Aa noted, some components will fit some other helmets, although, the manufacturers will probably discourage that.
Philbert