I've looked through as many sources as I could, but it seems the best I can get on a non lightning mace build is 15-20. Is there any way to get it wider?
I believe you should look at the Disciple of Dispater for crit builds. Levels 4 and 8 give an increase to threat range that stacks with the improved critical feat. This is the text:
Iron Power (Ex): When using an iron or steel weapon, a 4th-level disciple of Dispater gains a +1 insight bonus on attack and damage rolls. Furthermore, his threat range is doubled as if he were using a keen weapon. At 8th level, the insight bonus improves to +2, and the threat range triples. This ability does not stack with the keen weapon quality, but it does stack with the Improved Critical feat.
It is 3.0, so it might be frowned upon if the GM doesn't like 3.0 sources.
One other source is the Weapon Master, also 3.0. At 7th level, if you have the improved critical feat, you gain a +2 to threat range for your criticals. Text as well:
Ki Critical: Gain the Improved Critical feat for free. If you already possess this feat, add an additional +2 to your weapon of choice's threat range for critical hits.