Get something with goretex - it is the only fabric which is lightweight and properly waterproof rather than showerproof that won't make you sweat. And it is machine washable. Try Cotswold online, (or another outdoor shop) it is a hiking shop but I find that generally the coats are better made for walkers than riders most of the time. Musto are about the only horsey brand that use technical fabrics, the others are way behind other outdoor sports.
if you are after a cheap goretex jacket then go to your local army surplus store, good quality coats at bargain prices (admittedly not a huge range of colours!)