I would definitely visit the following countries before getting married.....
Dominican Republic
Panama
Brasil
Costa Rica
All four, atleast 3 times. Colombia is cool, and has some nice and beautiful women to meet and marry, but the four counties above also present many possibilities for a single man beginning or continuing his search.
Also, I truly believe that as along as a guy is outgoing and willing and able to initiate conversation, then he could probably still do well even without a marriage agency.
Aaron