Try the folks at Dimolex Corp., La Crescenta, CA 91214.
Their number is (818) 957-7001.  They make membrane keypads
that are very flat, in layouts from 2 to 128 keys.  They
have standard models, tactile models (with stainless domes
under each key to make a 'click' you can feel), as well as
backlit models.  Some of them can even be cut with scissors
to form a funky shape other than a rectangle.  Many of the
models are available in a 'kit' which includes a bezel,
colored and plain key covers, and rub on lettering to make
your own layout.  One piece prices aren't cheap, though,
as they want $10 for one four position pad *kit*, and $45
for a 40 position *kit*.

I have no affiliation with Dimolex or any company connected
with them.  I *have* purchased a couple of keypads from them,
and am pleased with what I got.