Colombia is often regarded as having the clearest spoken Spanish in Latin America, this makes it an excellent place to stop and organise some private tuition. You can find advertisements for private tutors on the message boards of most hostels (Casa Viena, Hotel Holiday or Hotel Familiar, all located in Getsemani). You are also likely to meet other volunteers in these hostels. There are now a couple of language schools in Cartagena too, although these are significantly more expensive than organising your own lessons on arrival, in general private tutors will be much cheaper.
Details of a few options are given below.
Jacqueline Mendoza (Private Tutor)
Jacqueline is a young and well studied Spanish teacher. She is experienced and flexible and it's great fun studying with her. You are free to design your lessons according to your needs, ideas and your Spanish level. That could include for example visiting museums or markets as well as discussing Colombian politics or other topics in a cafe. To ask your questions or arrange your lessons write or call Jacqueline directly. You can communicate with her in Spanish, English or French.
Amaury (Private Tutor)
Amaury is an elder private teacher, with whom you can communicate in Spanish, English or French to arrange your Spanish lessons.
Nueva Lengua (Spanish School)
A large, well established language school with a variety of programs. Significantly more expensive than using a private tutor but a good option if you are only here for a stay.