How urgently is quite urgently? If it's just the motors you need, Ewan (Gimson Robotics) has a range (12, 18 and 24V) available on his website and he's pretty speedy with his dispatch/delivery.

If you need the gearboxes too, the Argos 12V drills are currently £12.98ea. I bought a couple the other day and they're still all-metal gears and a D-shaped motor pinion/gear. Really good quality at the budget end.