Depends what sizes you are looking for, and where in the country you are based.
Local Timber merchant, independent DIY shop, or Joinery/Kitchen manufacturer would be a good choice. Builders Merchants and large chain DIY shed not such a good choice.
If it is just a few small bits that will fit in an envelope, or if you are near East Yorkshire, then I may be able to help, as Joinery Manufacture is my job.
(If you call it Engineered Wood, then no one can complain, that you have dared to mention the, “Brown Stuff” !!!)
Mark.