How about stacking two plates of 1/8" CRS. This is the most
thickness I can get locally with 3" width. Online ordering is
out of the question as I'm not in the US. The shipping charges
overseas would kill me.

Not going to work. Two pieces of 1/8" are not as stiff as a
single piece of 1/4". And welding them together would probably
cause warpage.

Are you in an industrialized country? If there are local
machineshops, you might stop in and see if you can chat with
one of the machinists and see if they have recommendations
on sources.

As to shipping, the 36" piece is about 7.5 lbs. Yikes. The
cheapest postage is about $30.