Stock seems to be out of 501x shock towers in most UK shops - you might get one from overseas by the weekend if you ordered now... maybe...
But also try -
http://www.fibre-lyte.co.uk/
Its a standard part now for them, they do all the graphite 501x parts bar the top deck - not sure why they dont do the top decks atm, but there you go.
If you ask nicely, they MAY be able to machine you one out by Saturday morning, they have always helped me out if ive asked them for various projects in a hurry and they can fit it in.