Does the DC-3 passenger airliner have slats? The DC-3 is a very popular and widely used aircraft, yet has low speed tip stalling risks due
to the design of its wing, which at the time of its development represented the leading edge in drag reduction for larger and faster mono-wing aircraft in the 1930s.

Specifically, the swept and tapered wingtips, at high angles of attack, would stall first.

Did the original design, or later modifications, include slats to remedy this issue?
Is there any way to reduce this risk other than retractable slats?
Although it's technically possible, you aren't going to find slats retrofitted to a DC-3 (nobody would ever pay for it). 

But yes, in theory you could improve the DC-3's stall behaviour and single engine performance very cheaply with Vortex Generators which can be thought of as a "poor man's slat".  VGs increase stalling AOA by between 1/3 to 1/2 of what you get with a slat (say, about 18-20 deg, vs 25 with slats) depending on the airfoil.   

VGs have huge benefits on light twins, reducing stall speed, taming stall behaviour, improving aileron response, and often reducing Vmc to the stall when installed on the fin.  They are quite miraculous, although they can be a headache because they are usually glued on and if the kid you pay 20 bucks to wash your plane knocks off too many, you are grounded until you fix them.

I can't find anybody that sells a VG kit for the '3 although it does seem to be possible to install them on a wing with boots. Possibly the DC-3's airfoil requires VG placement far enough forward that the boots are in the way (each airfoil has a chord wise sweet spot and if they are too far aft they don't do anything).  Or maybe nobody has thought of doing it yet.