Jersey/K-rail barriers are a simple enough shape that I'd likely consider making a master and casting them up, myself. Since they could be done in plaster, just about any old material could be used to make the mold, even the cheap mold putties sold in craft stores for making soap (especially since plaster would allow you to vary and weather the surface easily, obviating the need for crisp detail retention in the molding material).
The website chromedog linked has a decent bit of paper terrain. The only stuff I've personally used (beyond templates) came from
http://combatstorm.com/download.html. I actually used the printouts as a skin over plasticard forms, just to make the pieces (dumpsters and shipping containers are all that I used) more robust, and it's worked rather well.
As for the assorted other bits you're looking for, I can't think of a mold source beyond Hirst Arts. Depending on how much you need, it might actually be cheaper to buy ready-made pieces. There's quite a few detail pieces like those produced for model train layouts.