First of all, if magic weapons are allowed I would strongly suggest getting a throwing melee weapon with a 18-20 crit multiplier. Scimitars, Falchions, Kukri, Rhoka, and Elven Curve Blades are all example base weapons. The best possible base is probably the monowhip (2d6 18-20/X3 light weapon) from Paizo's Pathfinder Campaign Setting: Technology Guide, but adding the throwing quality to whips, while RAW, is probably unallowed in most games. A chainsaw (3d6 18-20/X2 two-handed weapon) is nearly as good and more likely to be accepted.
Assuming you want a non-magic ranged weapon with 18/20 crit, the technology guide also lists the Nuclear Resonator, which is an exotic two-handed firearm sporting a nice damage profile and an 18-20/X2 base multiplier. It is a bit pricey, though, at 90K gp and has a ton of drawbacks, not least of which is its overly-impressive name, which is likely to make the unfamiliar reject it out of hand. It could make a good primary weapon but you'd need a backup to deal with forcefields.
I don't play with 3pp and I don't know anyone who does, so I'm not qualified to answer with regards to items from such sources. A kindly future editor or another answerer should add such suggestions.