I always thought that our local sawmill was cheap for oak, having bought plenty there, but he tells me that he has a client that comes to the area annually to stay in his holiday home, then takes a trailer load of oak back to the UK!
Before we moved here, my wife worked for a freight forwarding company. One of their customers kept a French oak t&g flooring there. We bought it for our lounge, hall and dining room, 22mm thick x 150mm for less than half the price locally.
I’d have a look on ebay. There are many postings on there with oak for sale. Reclaimed would be useful. Your carpenter probably has the tools to cut oak railway sleepers into planks. At £27.60 each for 2.6 metre lengths, it’s cheap oak!
I forgot to add that I made a ledge and brace door for someone that was burgled. It was just before Christmas and the door was wrecked. No chance of getting a non-standard sized door made in a couple of days. So I went to Chausson, the local Builders Merchants and bought some coffrage. Wood they use for shuttering. A 4m x 30cm x 25mm piece cost €13. It was enough to make one door. I stained it and had a good door for next to nothing.