I asked Wulfsport, but the only springs they had were for the 40mm [I think] 81 forks. They thought these were too tight a fit in the 38s and would bind. 38mm forks are bit difficult for springs as they weren't in manufacturers' model ranges for long. All the standard suzie springs are too soft and XR 250 springs aren't available in the UK. Race Tech do springs on demand and have a really good clear website, so last resort would be to buy a set from them
Progressive used to have stuff listed but their site is poor and I haven't managed to access their fork spring info for a while, not sure if it's still there.
As for the RM swingarm, what's the main advantage, weight, travel or handling?