That's a very pretty wood "ceiling" that you're showing in the pics. Its funny that in a boat the "walls" are called "ceiling" and the whats above is called "overhead"...
Typically, you end up making these strips yourself from your material of choice--and width of choice. They'll usually be from 3/8" thick to 1" thick and the width may be from 1.5" up to 4"; the wider widths are more difficult to fit since you end up custom cutting them quite a bit for the curve of the boat. No matter how wide, there will be a custom fit because boats aren't square boxes
You might wish to pick up a book by Bud Macintosh called "How to Build a Wooden Boat" that happens to have good info on joinery and ceilings too.
If you have a tablesaw, you could easily work with 2" (8/4) x whatever width wood stock--you lay that board flat on the table saw, run it through "ripping" your strips to be 2" wide by say 1/2" thick (or whatever thickness you want). Like cutting slices of bread from a loaf. What's nice about using 8/4 stock is that it is relatively cheap, usually its flat sawn (see grain pattern on face) and you end up with the nicer vertical grain once you've sliced it perpendicular. Depending on the saw and the particular wood chosen, you might have to then run it through a planer to get a smooth surface on one side. Also, typically, the edges are "eased" with a hand plane or run through a router with a round over bit. You could do the "ripping" with a good bandsaw as well--but even though a table saw wastes more wood, its faster.
So, lets say you don't want to do that ripping work...figure out what kind of wood you want (teak, cedar, mahogany?) and go in search of that wood. You'll want "clear" stock, btw. Typically the stores that sell these types of wood also offer millwork services and you can specify what you want. So, you'd be buying your wood from them, lets say 8/4 x random widths x random lengths and having it cut to 8/4 (2") x 1/2" strips. Since they charge for every pass through a machine and you'll be hand fitting these into your boat which will require some custom cuts by you anyway, it might be most cost effective for you to pick up a hand plane to ease the edges. Also, you'll tell them that only one side will be seen so they'll only finish that side to the smoothness you need for a good painted or varnished finish.
If there's a Dixieline lumber near you, they'll likely be able to do the millwork but will not have the wood you want. You can usually take the wood you've bought elsewhere into the Dixieline and they'll cut if for you for a price per linear foot. Cabinet shops might be able to get you the wood. You'll find local shops that can do the milling work if you look in the yellow pages for "millwork" or under "lumber milling." Lumber yards usually can do it--but again, you have to find a good marine quality clear wood to start with. While teak is commonly used, it is costly and actually more difficult to keep a varnished finish on teak than some other rot resistant and pretty woods. Teak, purpleheart, silver bali, sapele, all kinds of mahogany, alaskan yellow cedar, port orford cedar...there are many woods that can be used for ceilings and overheads.
Good luck to you!
Some pics of us "milling" for our own Alaskan Yellow Cedar overhead:
we started with 8/4 x 6" x random lenghts 10ft -19ft and cut 3" wide x 3/4" thick--so a cut vertical and a cut horizontal then passes through the planer; yes that's lots of sawdust because we also used a dado blade to remove extra wood allow us to "spline" the boards together.
the overhead, looking up from below