its Hard out here MAX agencies normaly have guys they have used and have been requested for in the past these guys get the "pick" of whats going before guys the agencies dont know Its a bit of a bummer but thats life
The other way is sheer luck if your the first guy to phone after they ahve been asked to supply an electrician You could also set yourself a rota for phoning round and take names of who you speak to as hearing you on the phone at 11am every tuesday and friday may pay off