In reply to fire_munki:
Some friends and I bought some awful gaudy (women's) leggings from H&M to wear for a UV party at craggy island once..
As someone else mentioned, try Primark as well.
In both cases these will be more cottony street wear ones rather than sporting lycra, but for just a comp, who cares.
I'm sure you'll be able to find something suitably atrocious..