Wait for a set of used R1200GS or Adventure spoked wheels to be advertised. There was a set on UKGSer* ::::* For BMW GS Enthusiasts recently for £300 without discs, ABS rings or tyres. New pair of bare wheels are about £1,500.
Getting them built from scratch will probably cost £1,000+ assuming you can find someone to take the job on. Not sure if anyone in the UK will do it, only person I can think of is Woody's Wheel Works in the US, find him on Adventure Rider Motorcycle Forum. If going this route, consider having the hub laced to a tubeless 21" front rim from an airhead GS for increased offroad ability - same number of spokes etc.
Might be wrong here but if changing from cast to spoked wheels on a 1200, you may need the matching wheel bolts as I think there's a 5mm difference in length IIRC.
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