When wearing this tartan my advise would be to not limit yourself to the colors in the kilt when selecting what else to wear. Dont think just of the kilt, but of the outfit as a whole. You can match hose color to your shirt color, for instance, and because the black & white of the kilt is pretty neutral, you can wear just about any color you want.

I wear a Highland Granite kilt, which is just different shades of grey, and I'll often wear it with things like a red shirt, grey hose and red flashes; or a bottle green shirt, green hose and grey flashes.

Second point, this tartan should not be a special weave. It's the same as the Shephard check, which Lochcarron stocks in both 10 oz and 16 oz cloth.
http://www.lochcarron.com/tartanstrome/shepherd.html

If your kiltmaker or Highland haberdasher is telling you that the Northumberland tartan is not available, it's probably because they didn't see "Northumberland" on their tartan list and didn't know enough to realize it's the same as Shephard. So if they say it is not available, just ask for "Shephard" and see what they say.