You might net 10-15hp, should probably be tuned for, the deletes simply improve flow to the heads, but nothing to necessarily jump all over unless you were doing a new manifold or already doing work to the car IMHO.
Your really only going to see improvement if your shooting for big power. I remember a tuner shop doing some tests and found no gain in performance on a stage 2 car, as for where to get them..... aps makes a set but is big $$$. If you watch forsale threads on other forums, people sell their modified ones for cheap or do what rex said and check out ebay. Just be carefull because some people getto them and use jb weld to seal up the holes...... I've seen a one suck the jb weld right into the motor.