Browsing through SJS, some of Surly's forks have 300mm steerers but it's not listed on every fork:
http://www.sjscycles.co.uk/surly-disc-trucker-fork-1-1-8-inch-steerer-for-26-inch-wheel-black-prod30113/ http://www.sjscycles.co.uk/surly-long-haul-trucker-fork-1-1-8-inch-steerer-black-prod18316/
They also have an unpainted 700c fork with a long steerer for £10, might be worth getting some bosses welded onto that, if the legs are far enough apart for your chosen tyre.
@Emyr started
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Browsing through SJS, some of Surly's forks have 300mm steerers but it's not listed on every fork:
http://www.sjscycles.co.uk/surly-disc-trucker-fork-1-1-8-inch-steerer-for-26-inch-wheel-black-prod30113/
http://www.sjscycles.co.uk/surly-long-haul-trucker-fork-1-1-8-inch-steerer-black-prod18316/
They also have an unpainted 700c fork with a long steerer for £10, might be worth getting some bosses welded onto that, if the legs are far enough apart for your chosen tyre.