Go to Target and look at their women's tights. Cheap (they run for less than $AUD20), made of cotton elastin and likely in large large sizes. Don't be put off too much by the cotton content. I'm at the lowest end of the Clyde scale, but I've used these tights for years in all sorts of weather on all sorts of rides (commuting to 1200km randonnees) here in Australia and elsewhere, and they work.
If Target's sizes don't run to yours, look for other department stores that have extra-large sizes for women.
This also is assuming that you don't mind the idea of "cross-gender" dressing.