Database for NACA airfoil polars
The technical report NACA-TR-824 has been digitized.  The digitized version includes Cl and Cd for 118 airfoils at 3 Reynolds numbers.  To get it, create an account on x-plane.org.  (It's been online sporadically elsewhere over the decades, e.g. here.  People associated with the digitizing include Gregory Peter, Gregory Siemens, and James Sonnenmeier.)

Convert plots to tables of numbers with Data Thief or WebPlotDigitizer.

At lower Reynolds numbers, http://airfoiltools.com/polar/index lists XFOIL-calculated polars for many of those airfoils, as text as well as plots, e.g. http://airfoiltools.com/polar/details?polar=xf-n0009sm-il-500000:



alpha    CL        CD       CDp       CM     Top_Xtr  Bot_Xtr

-13.500  -0.9606   0.06449   0.06155  -0.0264   1.0000   0.0073
-13.250  -0.9745   0.05737   0.05426  -0.0319   1.0000   0.0074
-13.000  -1.0011   0.04929   0.04597  -0.0377   1.0000   0.0072
-12.750  -1.0177   0.04406   0.04052  -0.0404   1.0000   0.0073
...

You may need some clever webscraping script to collect them all in a NN-friendly format.  Or you could run XFOIL yourself:  they describe exactly how they generated these tables.