i am looking for a source for a pair of ribs for a sxs rifle project. both 18 in. long 9/16th-5/8 in. wide 3/16th-1/4 in. thick.

Galazan used to have them.

I too am looking for some ribs for a double rifle I am building this is the only source I have found. It seems all the others have dried up at least in the U.S.
http://www.bundukipublishing.com/EBCS%20Home.htm

usps
I am assuming from your post you are making your quarter rib and top rib separate pieces. If you have a milling machine you can make your own ribs very easily. Purchase some DOM tubing in the size that matches the radius you would like for your top rib. Set it up on the mill and use a slotting saw to cut out a section the width you want. You can make them tapered this way as well, it just requires a little more set up. Its fast, easy, and inexpensive.

usps,
I just re-read your post. I did not notice the exact sizes you had posted. I did not realize you wanted the thickness to taper as well. However they can still be made the way I mentioned. Just use heavy wall tube. The taper would require a little more machining. I would taper the tubing in a lathe and then cut out the ribs.