Pricers for Tufnol/Paxolin in various thicknesses on e-bay are quite reasonable.
For example,TUFNOL Whale Sheet Phenolic SRBF (synthetic resin bonded fabric), A4 sized sheet x 1.5mm thick, £5.00 post free:
https://www.ebay.co.uk/itm/P1-SRBP-B...-/252426091086
I've used a company called Trent Plastics several times. They often sell sheets more cheaply with surface blemishes such as scratches and sometimes call it 'refurbished' or 'used', but it's fine where appearance isn't an issue. For example, 300mm x 300mm x 2mm thick Whale sheet, £10.39 post free:
https://www.ebay.co.uk/itm/TUFNOL-Wh...-/162272259505
They have other thicknesses available up to 50mm thick.
Phenolic sheet sticks really well with 'original' Gorilla Glue (polyurethane).
Hope that's of help.