Check dental supply dealers, schein, Patterson,benco,Atlanta dental, your local dentist has catalogs with it in them and can order
They have them (see below) at $14.80 (Not sure about US cost of shipping but it was free shipping to the UK).
NORMAL PRICE I spotted in a US supplier is $50.00!!
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20180121025046&SearchText=Dental+lab+Heating+tube+for+Vacuum+Forming+Molding
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