Yes, I had a look at the Pololu options but they are also quite pricey.
Now, regarding not being able to guarantee equal current, how much of a problem is it going to be? The question really is, does a chip getting hotter increase or decrease its impedance? If it increases it (as in the case of resistors), the system self regulates. If it decreases it, we will have runaway temperature increase and the chip will blow, then the next one in a cascade etc until the while piggybacking unit is gone. However, L293s also have thermal shutdown, which probably means that the moment the hottest one shuts down, we will get a cascade of shutdowns. Annoying but far from catastrophic.
So, my question I guess is, what's the worst case scenario? A few blown chips are hardly a problem, L293s are dime a dozen. The motor can't be damaged by anything the chips do. In the unlikely case a chip blows (if the thermal shutdown fails for some reason), could it dump the input voltage (12V) somewhere where it could do something awful (like the Arduino PWM or GND pins)? Is it reasonable to protect against this by branching some 5V Zeners from each of these lines to ground as a safety valve?