I bought some used DVD's and then the work books from Ozone. A lot of the drills can be found in many books, on line, etc. What is nice is the book can be used at the table to set the drill up and the DVD shows how to do it.
I also have the Kinister series. Many of his tapes are one hour of showng a pro trying to do a drill. It gives you a appreciation of how hard the pros work and gives you the confidence of not giving up because the drills take time, patience and dedication. Since I started to do them my game has improved.
I think if you have the money to spend you will not be dissappointed. You can also recoup about 3/4 of your money on the resale market if you don't like them.