I had to drive around 1500km this past weekend in a rented mini-van. I don't own a car, so that's probably more distance than I've covered in a motor vehicle in the past 5 years.
Today, back home and on my way to work, there was rather torrential rain, but I was still so thankful to be back on the bike.
Just go to any local outdoor gear shop and get ones that are light weight gore-Tex or equivalent material. Though I will say I am somewhat of a freak in how little I sweat, so your experience might differ from mine. They definitely will not be as breathable as normal pants, but better than being soaked and wet. Helps if you cycle at a more leisurely pace.