I'm not very creative, so 'm not that great at scratch building terrain. Now, I love GW's 40k stuff, from the bastions to the ruins, but they have so few stuff, anyone have any websites for ordering 3rd party terrain that would flush well with the GW stuff?

Gale Force 9 do their Battlefield in a Box series that are excellent, and pre-painted. We use lots of them in our 40K game.

Pegasus also do some nice looking terrain. The Rivers are pre-painted, not sure about the buildings. Some of the Pegasus models can be seen in the MWG Batreps.

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NOBZ!

Try Ebay. There is some awesome terrain in a vast range of prices. You might also post this question in the Terrain forum; there are several people who build terrain for sale.