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Does anyone know a good source for it? Everything I have found is too small.
Yes, I know cardboard works too, but I was hoping for something less flimsy and easily ruined.

Cardboard is what I use...if its too flimsy you could cast it using a mold (my hobby shop has some white/transparent casting materials in blister i forget what its called) you can then make your own casting using miliput or green stuff...

What is too small? The sheets or the scale of the pattern? I have some O scale/1:48 corrugated siding from Plastruct which has ~2mm wide beads and came in a 2-pack of (just shy of) 7x12 sheets. I think that scales with 40k models much better than most corrugated cardboard and the sheets are far larger than I'd ever need for a single, continuous piece.

Cardboard is much too large for 40k scale. http://www.hobbylinc.com/htm/evg/evg4530.htm sells a few different sizes of corrugated styrene, figure out exactly the size you want and get it.

Plastruct, JTT and Evergreen both sell dozens of sizes of plasticard with a corrugated pattern to it. You can even get metal from companies like Builders In Scale.

AH! I don't have any idea how I managed to miss all of these.
I thought I was searching for corrugated siding, but maybe my search criteria was bad.
Thanks for the link.
What scale do you guys like?