Ealry Learning Centre sometimes have Dickie brand Vehicles but rebranded for ELC, I've seen the Dickie Unimog fire-engine in my local store.
Smyths toy wharehouses are another good source of 1/24th scale (or there-abouts) plastic vehicles, again a lot of them look like re-branded Dickie toys.
Also seen a few in my local Tesco and Asda stores but usually seasonal (around Xmas in particular), and as Mel says check out your local TKMax.