Help need to find a dress quickly

Hello everyone,
This may be a sensitive topic and I do apologise in advance if you are affected by my post.
We have recently found out that my partner's mum is extremely unwell and has only a short time left with us.If we are able to organise the registrar we could be getting married as quickly as Monday the 11th. I won't be wearing my dress as it wouldn't be ready. I'd love to find a simple tea length dress for our ceremony. I may be asking the impossible but does anyone know of anywhere I might be able to get something?It doesn't have to be a bridal store, just somewhere I can buy off the peg.
Thank you in advance