HPAC should be your first point of contact
http://www.hpac.co.uk/paintball.htm
They offer all levels of service, from equipment hire to fully manning and running air for events
There are other people who can offer compressor hire, however you must ensure first that is the appropriate compressor system for paintball - providing breathing quality air, not industrial high pressure air. Plus there have been cases of events getting let down by suppliers not being able to cope with demand on the day or even cancelling
Make sure whoever you get knows your requirements, HPAC will most likely be happy to discuss and they know the difference between what an organiser thinks they may need and what the usual capacity tends to be