For any 5’ wide Avid users in central California. Macbeath Hardwood in Stockton CA now sells 5x8 and 5x10 MDF. (3/4"). Nice not to have to piece together a spoil board.
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Next time you head up to Yosemite, let me know and I’ll give you my list! 
For those in the Southeastern USA, A&M Supply not just sells 61"x121" MDF but also carries those sizes in UDF for your flow through fixturing (some folks call them “vacuum tables”). I have found it very difficult sourcing the UDF, especially in that size.
If you are looking for the same size in HDPE, sadly they are only cast at 60"x120" but again, in the Southeastern USA;
https://www.piedmontplastics.com/products/hdpe
(https://www.piedmontplastics.com/resources/literatures/king-cuttingboardR-xl)