Adam W.
A Major Clanger
Modern timez, innit!As others have said, suppliers typically sell in fixed thicknesses and the smallest standard thickness you can probably use is 2 inch / 50 mm.
The discussion about work and machine wear to get the material down to your required measurement is interesting because I often don't do it that way. Getting down from 50mm to 38mm is going to be a lot of work no matter how you do it. I avoid working to fixed dimensions as much as possible.
I use hand tools only. Very often I find the thickness of the piece is quite arbitrary and it doesn't much matter how thick a part is. So long as it is thick enough to be strong enough and any extra thickness doesn't adversely affect the looks, that is OK with me. For example, I am currently preparing some table aprons that are coming out thicker than I need but what does it matter? Nobody will see that unless they climb underneath the table, and I am far too lazy to plane a few mm off just to reach some pre-determined thickness.
I expect the same thing will apply to many parts of your bed, so I would reduce the work required and the wear on your machines by just removing enough material to get the boards flat, straight & smooth (enough). What harm would it do if your finished pieces were 44 mm instead of 38 mm? What harm would it do if the left side of your bed was 43 mm and the right side 39 mm?
Or am I the only lazy sod here?
Once upon a time, in the not too distant past, material thickness was nominal and differences were accommodated for in setting out and construction.
Today in machine controlled carpentry and joinery, 38mm is 38mm and nothing else will do.