Besides the other suggestions, you can go to the source. Three of my favorite electronic enclosure manufacturers are Hammond, Bud and Polycase. Polycase in particular has pretty low setup and per-unit costs for modifying enclosures.

Here's the UK version of Farnell, who are "quite big" in component distribution.

http://uk.farnell.com/enclosures-racks-cabinets

There are several thousand off the shelf boxes here.

Google "electronics enclosures". Or look on the distributor's sites (Mouser, Digi-Key, Element 14, etc.) where you buy other electronics parts.