Shwmae, Steve, croeso.
I’m not sure what there is in Gwynedd or the North-Wales A55 strip in the way of hard-wood suppliers, but builders merchants and the various DIY sheds are to be avoided – the offerings are usually expensive, poorly sorted and unsuited to quality work.
Your nearest best-bet, I suppose is to make an occasional trip into England, the Chester or Liverpool areas to stock up where you may find more specialised suppliers, but it will need some research and ringing around to see what stock is available from time to time.
There are some suppliers of hardwood on line, but when you factor in delivery charges, it can get very expensive and can never compare with selecting your own.
I’m in Ceredigion and I have an informal arrangement with a local maker who buys hardwoods wholesale and adds my occasional stock to his orders….. you may try to seek a similar arrangement on Ynys Mon.