Moving to barranquilla

There is no licensing or knowledge requirement for "real estate agent" in Colombia, and nothing equivalent to "realtor" as in the USA.
There are real estate companies that handle bienes raices who will show properties for rent and for sale.  You can do searches to see on-line listings:

barranquilla bienes raices

barranquilla alquiler de apartamentos

barranquilla alquiler de casas

Here is a Colombian equivalent of Craigslist, where anyone may post free ads, so owners may post as well as companies.   Look under INMOBILIARIA and then the first line under it is for apartments, and the fourth line is for houses:

https://barranquilla.locanto.com.co/

You may find it difficult to find a place to rent, and even more so as a foreigner without a cédula, the national ID card. 

Here's how to get a cédula de extranjería.  Note you must first have a visa...

https://medellinguru.com/cedula/

Here's what to do to get a visa:

https://medellinguru.com/colombian-visa/

Landlords are wary of renting to people for which there is no recourse to collect rent, should things head south, because in Colombia it is hard to evict undesirable tenants.  You may be able to convince a landlord to rent to you by paying rent in advance - up to 6 months may be necessary, depending if you can get a landlord to trust you.

At first you may want to stay at an AirBNB place, where you can often negotiate extended stay rates, and no visa or cédula is required:

https://www.airbnb.com/s/Barranquilla--Colombia

Thank you OsageArcher! This was extremely helpful!