Hi Hunter,
You've certainly hit upon a major gripe of the educated diamond shopper. Indeed, many a diamond is for sale without some of the information that would be helpful in determining the mathematical optical performance of the cut, but there are some options available to the determined consumer.
First of all, while GIA reports don't include the relevant angles, AGS reports do contain average crown and pavilion angles so perhaps that will help you. I think it will be only a matter of time until GIA starts including more of this information on their certs (they already do the Sarin analysis for the stones they grade), but in the meantime AGS enjoys an advantage in this regard. Both GIA and AGS certs are extremely respected within the industry so either should be reliable for you.
Another thing you can look for is detailed Sarin or Megascope analyses for stones you are considering. Many online vendors include these reports with the stones they list and an increasing number of bricks and mortar dealers are starting to see the benefits as well. These reports will give you all the information you need.
Most jewelers are ill-equipped to provide this information, but some will be willing to obtain the reports for you on specific stones they offer. They won't want to send a boatload of stones out to be scanned, but if you have a specific stone that you really like some jewelers will have the diamond scanned for you if that's what it takes to make the sale.
-Tim