I use a light waterproof jacket, light over trousers and overshoes when its wet, I just ride easy to stop me sweating up, I also have one of the old fashioned capes I sometimes use, I cant help the OP with where to buy it from though, I've had the waterproofs for ages. I brought the light jacket of the internet, the over trousers came from a place that specialized in industrial kit and the overshoes came from the LBS, I walked in wearing my steel toe capped boots and asked for waterproof overshoes to fit over the boots, that was a while back but the overshoes fit well and have kept my feet dry on many commutes.