Hi Bob,
You can get a new 17" rim and spokes from central wheels in Birmingham. They can also supply an 18" excel or SMPro rim and spokes. It's who I use to get my rims & spokes from. I had a quote off them the other week for an 18x2.5 excel rim and spokes for a DR650 and it's about £155 for the rim and about £80 for the stainless spokes (if that helps) - the same for the 17x2.5 rim.
Regarding 18" conversion, I laced an 18" to my tenere last year up from a 17" to see what it would be like. No noticeable difference to road handling. Off road, there is a very minor difference in that the bike seemed to track ever so slightly better. For what I do, I didn't really think it makes much difference between the 17 and 18 rim to be honest. The 18" rim is useful for more knarly green lanes as you can fit more off road focused rubber - but to be honest I tend to stick with the same tyre I used with the 17" set up (mitas E09). If you were thinking about rallying the 650 or taking it on more knarly tracks then the 18 seems a good idea as you can find a wider variety of enduro/off road tyres.