Honestly, I don't think most alkaline battery companies will tell you. Its a little frustrating.
Some of the 'name brand' battery companies do have datasheets with such information, but they can be difficult to locate. However such ratings are usually expressed at some small current discharge rate and if your actual current draw is at a higher value then that, then the equivalent mAh rating will be lower. Especially for those small 9 vdc batteries, that are really designed for low current draw devices like smoke alarms and the such.