Generally speaking you'll have fewer scheduling issues if you book an appointment at least a couple of days in advance. Some providers are low volume so getting a time set up isn't an issue but others go the route of quantity over quality and their schedules can be very busy.
If you're looking for a spur of the moment appointment I recommend checking out the SNATCH thread, linked below for your convenience.
http://www.eccie.net/showthread.php?t=1195043