"Nashville tuning" is where the high strings from a 12-string guitar are put on a regular 6-string guitar. The tuning is the same as a regular guitar, from low to high, E - A - D - G - B - E, but the E - A - D - G string are tuned an octave higher, requiring a smaller gauge of string. The E - B strings are tuned normally.
This is a list of the ready-made acoustic guitar sets for Nashville tuning that are currently available (with links to Amazon). Just to confuse you further, the strings are listed here as sold by the manufacturers, in high to low order (i.e. E - B - G - D - A - E):
.010 - .012 - .008 - .013 - .017 - .025W (C.F. Martin MSPHT-10) Phosphor Bronze (Buy On Amazon)
.010 - .014 - .009 - .012 - .017 - .025B (SIT P1025NT) Phosphor Bronze (Buy On Amazon)
.010 - .014 - .009 - .012 - .018 - .026W (D'Addario EXL150H) Nickel Wound (Buy On Amazon)
.010 - .014 - .009 - .012 - .018 - .027W (D'Addario EJ38H) Phosphor Bronze (Buy On Amazon)
.011 - .015 - .009 - .011 - .015 - .020W (GHS CU-BBT) Bright Bronze
.012 - .016 - .010 - .014 - .020 - .030W (EmStrings AC12PBMED 12-string set) Phosphor Bronze
.012 - .016 - .012 - .016 - .023 - .030W (Pearse)
.013 - .017 - .013 - .017 - .026 - .035W (TBA)