The Ashcrofts of this world will do you a new/recon 4-pin, but I don't know how much. Otherwise its a case of keeping your eyes open. I have one but its in the car.
Equally a Detroit Truetrac or similar, probably is £500 by the time you've had it built, but I don't think the component parts are much more than £350.
Getting a standard diff professionally built or pegged is another option that might buy you some longevity, but 11 is quite alot!