Woodland Scenics "Scenic Cement" is based on acrylic matte medium which dries flat rather than gloss, which is what white glue dries to.
I have mixed matte medium with water to a 40-60 mixture with a drop of wetting agent and that has given me a mix almost identical to scenic cement.
That being said, I find that white glue mixed 50-50 with water works every bit as well as scenic cement or matte medium for most scenery application as most scenery is not affected, appearance wise when it dries.
Also, white glue is much less expensive, as I get it from the local building supply store by the gallon.
If you wish to use matte medium, you can get it from any artists supply store.
Happy modeling and Cheers!
Terry