Can you buy an inexpensive rectifier/dc power source for cheap flickering LED Christmas lights?
No-one will sell a commercial product like that.  It's dangerous.

DC is nasty business. Seriously, go to mouser and pick any random power relay.  Look at its specs for AC, and then look at its specs for DC.  Whoa! 

DC is de-rated by a large factor because when a DC arc strikes, it doesn't self-extinguish because the voltage never goes to zero.  An arc, once struck, doesn't feel pity or remorse or fear and it absolutely will not stop until it's burned up so much conductor it can no longer bridge the gap, and if that sets your house on fire, too bad.  If you've seen old "snap switches" which have a considerable action to them, they are preloading and releasing a "snap" action which creates a large gap when the contact breaks. Larger switches and contactors have designed-in "blowouts" which bend the arc into an arc-chute.  The point is, interrupting DC takes a lot more design and engineering than interrupting AC. And a short is a much bigger deal. 

Now when you rectify AC into DC, you get a bouncing-ball waveform. 



Supposedly that should self-extinguish same as full-wave AC, but capacitance will prevent it from getting all the way to zero.  If you add a smoothing capacitor, then it has no chance to extinguish. You wouldn't want to do that anyway, because that would raise the average (RMS) voltage, which could overcurrent the LEDs.  LEDs are non-linear, so a small voltage increase makes a big current increase.  The curves are in LED spec sheets. 

Of course if you could constant-current regulate each LED chain, it's perfectly wired for that.