>> I need to create a stored procedure that will return all combinations that are not already being used. <<

This is not a good idea; do the math. In the old UNIX systems, we had
a password generator that created 6-8 letter words that were
pronounceable but non-sense. Compuserve used a two-word pass phrase
that was long but easy to remember -- "purple bagpipes", "vividly
moose", etc. Then there are "bingo card" systems that are quite
safe. Just what do you want to do?