B&Q, I got a (bright orange, shock!) plastic one from there at the start of the year. It's 90L capacity and £33. All the other liveries at my yard love it as much as I do, so it's very in demand and has stood up to the job really well.
The Orange plastic barrow at B&Q is good value & very practical. The frame is aluminium, so it's light. It has a pneumatic tyre so is easy to manoevre & it is designed to carry the right amount. If you get a barrow that is any bigger you tend to put too much in it & it becomes too heavy to push. We have to negociate a ramp to a large muck trailer & anything bigger we have difficulty pushing up the ramp.