Do you have to wear a hat or anything? That would be my worst nightmare about an event like this!! Well, that and wearing heels.
What sort of event is it? Do they give a dress code?
Sometimes places like John Lewis, Phase 8 etc have a lot of older season stock on sale, but it's not shown in store, just when you search for things and use filters for size/brand etc - if you are lucky, you might find something in the right size that is on sale still.