You should know where to get them refilled if you are having them serviced every year as required.
But anyway, back to the matter in hand.
Look in the yellow pages and on the web. There will be loads of companies.
The big nationals will be good but dear. The really small firms, one man bands etc will be cheap but are often poor at maintenance (but then again some large ones are - Protec spring to mind).
From experience, medium sized local/regional firms that are members of trade associations and/or have BSI accreditation are normally the best trade off between quality & price. They are also, unlike smaller firms, more likely to have their own CO2 filling rigs with a quicker turnaround.
Your message implies you are doing training within your own company. If not and you are a training company & if you don't mind offering extinguishers for sale (not servicing) as a sideline you can then get a trade account with Thomas Glover or UK Fire which will then mean you can get filling done at a far cheaper price as long as you can cope with a weeks turnaround
