Stay away from buying them on the street — they are likely fake. Instead go to Casa Havana for a cigar, a mojito and some live salsa.
If you're into them, this is the place to get them as Colombia is the world's largest producer of Emeralds. From what I've heard, Lucy Jewelry is the place to get them.
These bags are literally sold everywhere. They come in bright colors or neutrals and can range in price from $10 to $50 depending on where you go. I got all of mine on the street and they made great homecoming gifts.
THIS IS CARTAGENA
There are bunch of these around the city and they sell local wares all produced in Cartagena.