Pure Bash (no external utilities), 57

printf -vn %0$1d
a=${n//0/{$2\}}
eval echo ${a//\}{/\},{}

Input is given as command-line parameters; 1st is n, 2nd is a comma-separated list.

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Pure Bash (no external utilities), 57

printf -vn %0$1d
a=${n//0/{$2\}}
eval echo ${a//\}{/\},{}

Input is given as command-line parameters; 1st is n, 2nd is a comma-separated list.

printf -vn %0$1d         ;# Create a string of n "0"s in the variable v
a=${n//0/{$2\}}          ;# Replace each "0" with "{a,b,...m}"
eval echo ${a//\}{/\},{} ;# Replace each "}{" with "},{" and evaluate the resulting brace expansion

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