the 1/4 inch liner is a hard lumber to store without it moving too much (warping, bowing etc). So what you are actually getting when you order is the millwork to resaw a larger piece down to your spec.
If you order online, Woodcraft is likely to land at the top of most folk's list for the liner material. I have used them once and was happy with them. I currently use a Woodworker's Source here in Phoenix. Their in-person prices are better than their online prices and i get my lumber resawn and ready within two days of calling in my order. If you can find a place local who has the Spanish Cedar, they would be the best to strike the conversation up with.
For your specific project, I would stay away from the wider widths anyways. If you were to find a cedar liner in a wardrobe or other cabinet from that timeframe it would be quite common to see it with a liner composed of smallr strips with marine lap type of joints... were it me, that's the way i would go. I think the extra texture inside would be interesting. (just my .02)
Can't wait to see your project as it comes along. Good luck on your quest!