Schneider make absorptive ND filters up to ND 4.0 (13 stops). These are foil sandwiched between glass - the foil is, as far as I know, a Kodak Wratten gelatine filter. They also make reflective ND, but these are also glass. They both come in 4x4 as well as other sizes.

Anyway, the obvious thing is to get the Wratten filters, if 13 stops is enough for you. Here in New York, Adorama have a 4x4 ND 4.0 for USD 72.

As far as I know, Lee polyester filters only go up to ND 0.9

Best,
Helen