Not sure about waterproof trousers but for jackets I always go to Tk Maxx. They have proper outdoor wear at really good prices (I paid £25 for a ski jacket, which has zipped vents under the arms which is great for riding), tends to be mcuh more waterproof than horsey stuff.
I'm a fairweather rider but my pony is even worse than me! He's a right wimp if I ride him in even the mildest rain and tries to turn his head away at every opportunity! I need something to rain proof HIM!!
I don't think you can beat old fashioned stinky wax for waterproofness! we had a lesson today in wind and rain, I wore wax fleece lined chaps and a old long toggi wax coat with leg straps, and not only was I totally dry but so was my saddle. He wore his equisafety wrap around sheet and again only a wet neck for him. Wax stuff could be picked up second hand cheaply as it's a bit old fashioned now, but it works!
These .... i had them when i was working out in the field at DEFRA, and i have them now where i work too (though they are high viz ones where i work now lol) ...... I wear mine for doing my horses in .... and they work a treat, totally dry legs