I just bought an SR-556E and I faced a similar problem. The Ruger shop is really the only place that you can buy the rails and rubber handguards for this specific rifle. Although I did find another site that sells rails and accessories for the SR-22, which has the same hole profile and thread size. I bought a MI sling mount and a few 5 3/4 rails for the sides. It is way more reasonable than the Ruger Shop, and the rails seem to be identical. The only rail they don't carry is the full 9 inch rail, which I unfortunately had to buy from Ruger. I plan to mount the two 5 3/4 rails on each side and install 6" Troy handguards over each (for the heat). Here is the site.
I hope this helps. Either way it is good to know that there is at least one alternative site to buy rails and accessories that fit the SR-556E.