SFGate

JAMEITE Hand-painted Silk Wallpaper Co., LTD Noble temperament,i like it.

Tres McKinney Design Get thee to your local lumber store, with picture in hand. It is plain flat stock and should not be a problem to find.

FN Design You could go to your local hardware store, buy some 5/8 mdf sheets and have them rip a as many 5.5 and 3 lengths that you need, sand, prime and paint... voila you're good to go.

Ironwood Builders Go to the local builder's supply...Not the big box stores. they carry many different types of interior trim. Show them what you want. If they can't get it for you from a supplier, it can be made. Your desire to avoid custom, but want a non stock item is...counter-intuitive? That is square stock. They have that in spades. To make the base thinner it costs less than $0.50 per linear foot...or less if you need a lot. Ripping down MDF, means any wall over 8' long has a splice in it....and that is not so cool. Pros layout long walls first with their longest stock to avoid splicing. Sure they splice a 22' foot wall...once...not twice. So buy where the pros buy. One other caveat to MDF...any sawn edge will need to be sanded and sealed prior to install. It will still soak up paint faster than the factory edges and you may never get it to look perfect. It is cardboard after all.

Ironwood Builders One more thing. Casings...the trim around doors and windows, is thicker than baseboard so that the baseboard has a reveal to the casing. Use 3/4" material for the casing and have the taller stock milled to 5/8" for the baseboard. An entire house of base board can be in excess of 1500 l.f. Expect to pay a
$100 set up fee for the job.

FN Design Agree with everything Ironwood Builders said... was just offering the 'cheapest' non custom option available... but you're suggestions are way better then mine. ;)

FN Design And I didn't mention anything about the casings being 3/4 because from the picture posted it seems even the casings are 5/8... kinda hard to tell.