Always the obvious choice is to go by word of mouth, if a business colleague of yours already uses a fantastic, can't be without Virtual Assistant then that's usually the first port of call.
Your local Chamber of Commerce or Business Link is a good source of information but (VA) Virtual Assistants are still quite new as a category in most business directory listings.
You generally need to look under business services, secretarial services or office supplies.
The Internet is your next search - Google terms like virtual assistant or virtual admin will straight away bring up a list of Virtual Assistants to choose from.
What specific help do you need with your admin? Have a think and write down what support you require - look at the different services offered by the numerous VA websites, there are loads to choose from.
Check out the pricing, not all VA websites show clearly what they charge per hour, plus some do monthly deals and contracts.
There should be testimonials on the websites, which enables you to make contact and ask what kind of service they get from the Virtual Assistant they're using.
Be direct and find out how long they take on jobs and what they're turn around time is on deadlines.
Last but not least, try out a VA, just a couple of hours support work and see what they can provide.
There are no tie-in contracts on an hourly rate, so you've nothing to loose by testing them out - remember you're in control of how much you spend.