It depends on how you want to recruit a babysitter.
There are national agencies, such as Sitters, Safehands, Abtot who operate like an employment agency - finding you a suitable person, charging you a finders fee / membership fee, booking fee or fee relative to number of hours you employ the person they provide to you.
There are websites that purely list evening babysitters, such as FindASitter.co.uk and BabysitClub.co.uk
Then there are job sites which are either general - such as manchester.gumtree.com, manchester.vivastreet.co.uk or are specific to a particular industry such as nannyjob.co.uk
In addition to these online resources, off-line/traditional methods of finding a babysitter are still going strong in many areas. Word-of-mouth is often a good method, just ask other parents who they use. Local newsagents may have postcard advertising, as do some other shops.
Hope that helps.