Finding a local reliable friendly babysitter is not always easy. My view is that you are better looking locally, then using a national agency if you are after occasional childcare. If you want to go out on a very regular basis however, one local babysitter may not be sufficient - thus agencies can be useful as they will know of several people in your area with luck.
I like many childcarers I suspect, started babysitting as a young teenager (it was better than a paper round). Nannies, Nursery Nurses and childminders can make ideal babysitters - though not all will want to be working daytime and then evening as well. Postcard advert in a local newsagents can sometimes work well to find someone local - as can asking around at school/pre-school/nursery. Word of mouth is always going to be best - though you may find that some parents won't tell you much about their babysitter, if they feel you may use the babysitter so much that they don't get their own nights out.
ChildcareLink and Children's Information Service alas will not be great for finding babysitters - as they refuse to list babysitters (at least in Surrey). Law changes in September 2008 - so with luck towards the end of 2008 more information about local childcare providers will be available to parents. Some childminders listed by ChildcareLink/CIS will offer babysitting - but a lot won't as they typically have their own young children to care for in the evenings.
Mumsnet of course is a good place to start looking for a babysitter - for example a babysitter in Small Business Ads , and in you can post an advert in Nanny Shares, though not a nanny share some evening babysitting may appeal to a nanny working in your local area.
Depending on how old your children are, you may want a babysitter who is trained in childcare, has First Aid training and anyone working with children should have an Enhanced CRB check. By using an agency like Sitters you can be reassured that they will have checked the babysitter's credentials - though always check yourself as well (can't be too safe) - whereas hiring a babysitter via private ads, you will need to do the paperwork checking yourself (it isn't hard, just ask to see certificates).
To provide any more help, I really need to know more about your family... age of children is important - not all babysitters have experience of caring for babies for example, location is also a factor - in towns/cities more likely to find someone than if you are rural - plus you need to think about how a babysitter will get to/from your home.
Also, how often you would use a babysitter and how for how long you would typically be going out for?