Try these:-
http://www.transmissionunits.com/
or these:-
http://www.commercialgearboxes.co.uk/
There are others - search on Bedford Truck gearbox repairs / axle repairs.
You
might be able to fit a two-speed rear axle (though I'm doubtful whether you could find one small enough to fit your vehicle - they tended to be fitted to larger models).
If you want to fit your rear-axle gear ratio:-
http://www.ebay.co.uk/itm/Bedford-TK...item233535f0fa
I don't know what ratio you have - it might be stamped on the axle housing (front casting).