I have mostly been unsuccessful, due to some belief by sports clothing manufacturers that only skinny people do sport. Many women's cycling clothes only exist in sizes up to a UK 14, with a handful coming in a 16, and virtually nothing in an 18. (I mean, seriously - what is up with a UK women's size 16 being classed as "extra large"? Size 16 is the modal size! Something like 40% of British women are that size.) Men's cycling clothes come in sizes up to XL or XXL, but would be completely the wrong shape for me (I am classic pear-shape - small waist, big hips, enormous arse) as well as being far, far, far too long.
This won't help everyone, but I found that Craghoppers do women's overtrousers in sizes up to 20, in both short and standard fittings. (Look at Womens Pakka Over Trousers - £30). I also found a cycling clothes supplier in the US called Aero Tech Designs, who have Big Man clothes up to 5XL and Plus Women clothes up to 4XL. Oddly, though, they don't do waterproof overtrousers at all. I should probably email them about it.
It just annoys me so much, because one of the great Myths about Fat is that fat people are fat because they don't exercise enough. Well, in that case, shouldn't clothing manufacturers make tons of big size sports clothing to encourage the fat people to exercise? How are you supposed to play sports if you don't have the right protective clothing? Grrr.