The first aftermarket block came out about five years ago. Aluminum, 6 bolt mains, capable of 3,000hp. Bare it costs $3,000 and still needs machine work.
Even now that there are options other than the Buick Motorsports Stage II blocks the prices still haven't dropped. You're looking at $10-15k on a motor.
Finding them (assuming you don't go the aftermarket route) hasn't got any easier, either. A lot of guys have a lot of Stage II parts they don't use and don't want to come off of.
Why not put together a beater motor until your "real" motor is ready? That's my plan and why I'm using the $500 slug.