I have improved critical so it is a 19-20 any to increase that? My search FU is very weak.
Barring you having a real stickler of a DM, or playing in a PFS game I'm not sure why it wouldn't be possible to use Keen or some variation on it that is created for bludgeoning/piercing weapons. While it's not Pathfinder or even pen and paper I know DDO has Keen, Impact, and Seeker. Each works for a different type of weapon.
Just an idea. As someone who only ever DMs it's something I would allow in one of my games.
Are you allowed to use 3pp materials? If so, Super Genius Games has a guide to critical feats that may help.
Keen does not stack with improved critical.