How many do you need, and what stage are you at in terms of taking this to market? Do you want a few dozen prototypes, or 10k ready to market cases?
Do you have a finished mechanical design for the case, or a napkin sketch? (are you looking for someone to do design work for you?) Designing cases for injection molding is not trivial, so you might be best finding someone who can assist on the design front.
If you are prototyping / doing pre-production units for user testing, then I would suggest rapid prototyping, as you avoid the huge tooling costs involved in injection moulded plastics. It lets you get prototypes fast, and iterate quickly. Downside is the structural strength and cosmetics are worse, however good prototyping shops will be able to sand/polish these models so they are nearly indistinguishable. Obviously this is no good for production.
Next up are prototyping injection mold shops, such as
http://www.protomold.co.uk. They have quicker turnaround times then most big places, and tend to take small orders (note I haven't used protomold myself, but have heard good things). They can turn around a design in around 15 days.
After that comes "proper" injection moulding, expect to have to sink in around £5k for the mould, and commit to 20-50k units purchase order. Also there is a significant lead time on making the tool, so budget 4-6 weeks to get your mold done.
To be honest this kind of question would get many more answers on cnczone.com then here, as its really more of a mechanical engineering question.