Hi MS,
The following suppliers sell mannequin heads, I'm not so sure if these would do, though. Pros Salon Supply's mannequin's hair, for instance, is only 16' to 18' long. Ameriglobe's are bald but with wigs.
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Now, the suppliers I've listed below might have what you're looking for. They aren’t in our directory, but they should be safe to deal with. They've been selling online for a number of years. They also accept safe payment methods such as PayPal and major credit cards, meaning you can get your money back fairly easily if something goes wrong. However, please note that we cannot guarantee the safety of any suppliers outside SaleHoo.
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All the best!