Rail advice needed....

I'm wanting to go to the Derby County fixture next Saturday,but i was wondering what's the name of that site where you can purchase any ticket for an rail trip?
It's not www.nationalrail.co.uk.
Also,it's from Letchworth,Herts to Derby, even if i booked it though midland mainline would it still be accepted and how long do these tickets take to arvive?

Cheers,and also,yes i've phoned up rail enquiries!

Posted By: Mick_Jagger, Nov 27, 21:36:04