10mm is an unusual thickness. You could try a local joinery shop, they could maybe plane some from 19mm floorboard stock if it's wide enough for your needs. By that I mean you could supply them with the boards, which you can get from floorboard suppliers.
Yeah I had thought of floorboards but I need 180mm wide for the window and 160mm for the door.
And yes, well aware how much the price of timber has gone up!
I'm not set on white oak. Larch could be a good alternative.
I want to fit some timber reveals to a window and sliding door I've just had installed. Does anyone have a good source for PAR 10mm white oak?