The best rain suit I found was one that was designed for motorcycles. Now granted, this is geared for pleasure/trail riding more than for shows. I have a suit I bought for my bike that's perfect. The pants have a grippy coating on the inner thigh that works well on a saddle. The coat covers them nicely and seals at the sleeves. You can ride all day and not be bothered. Plus there are lots of pockets.
Get the Ariat rain pants. Best evah! No slip seat and inner legs, wears well, very water proof. I've tried a number of them and so far these are the best by far. I have a vague interest in the Horseware style ones but I have friends who say they are slippery.