Filter manufacturers like B+W make high-density ND filters. Note that for filters denser than 8× (3 stops), not the factor but the logarithm thereof is specified.
So an ND 1.8 filter, for example, has a filter factor of 10^1.8 = 64× = 6 stops. ND 3.0 is 10^3 = 1,000× = 10 stops. ND 4.0 is 10^4 = 10,000× = 13 stops. That's about the densest you can buy off-the-shelf. These are quite expensive because ultra-high densities are hard to manufacture reliably and consistently, and dealers usually don't have these in stock.
And beware: Do not confuse ND 4.0 (13 stops) with ND 4× (2 stops)!