Where and how to get an AS-number?
What I need?
How much does it cost?
Well, first of all technical requirements:
Then an AS number is pretty much a formality. Given that you do not fullfill nr 2 (or you would know) you possibly also have no business having an AS number ;) Sorry.
(Assuming you are talking about BGP AS)
You probably should mention why you want this. If it is multi-homing, you might find it a lot easier to use a multi-homed provider and then peer with them using a private AS number. A good collocation data center likely will provide this, it is a good question to ask when choosing one.
If you do the peering to the Internet yourself, you are going to need memory for large routing tables, you will also need to make sure you route doesn't flap or it will be suppressed for a while, etc (In other words, it is not trivial). If you really do go this route, O'Reilly's BGP book is pretty good.