There really isn't. Nothing that matches well with the aesthetic at least. Lots of people do 15mm scifi terrain, or basic buildings, but no ones copied halo buildings.
They said they will be selling the terrain they have shown in their pics from last year at gencon and such, but no ETA.
Right now its the scatter terrain sets they sell and the 2 buildings in the 2 player set.
Cloest thing is Spartan also sells this set for planetfall. Though
id guess the scale will be off. Planet fall being 10mm, halo 15mm
http://shop.spartangames.co.uk/ProductDetails.asp?ProductCode=PFTB01