there are a gazillion ready to ship BBC crate engines out there... just get the one you want, if a 502 is just what you want they are great value, but once you start swapping ot parts its a waste of money... then its better to get one to your spec if you are in the market for a new engine that is...
this one was on my wish list till i sold by BBC car...
http://www.ultrastreet.net/engines/540_realstreet.asp
but a remanufactured 502 with a healthy cam is also a good option
you could also check
www.racingjunk.com there are usually some interesting ones for sale