Hi John; Welcome to permies.
I bought my ceramic fiber board from ebay here is the item # (222715872389) 1"thick x 19.7" wide and 39.4" long cost was $95.60 free shipping. Cut properly this is enough to do an 8" J tube using split bricks for the feed tube. I haven't ordered it yet as I may share a roll with another permie person but ebay has ceramic fiber blanket (FOR making a 5 minute riser) here is the item # (251999917103) #8 2300 F 1" thick x 24" wide and 25 feet long for $
93.00 ! 25' is way more than I need but if you buy 12' it costs over $80.00! Buy extra !
Good Luck with your material search and your future RMH build!