Asking for Information, the Right Way
You ask for a cost-/effort-effective solution to procuring insulin in Colombia. The fact that most authoritative websites on the topic only explain how to pack supplies, and how many to pack (see Diabetes UK for an example) somewhat hints to the fact that you are not guaranteed to be able to always find, and buy, insulin while abroad. Having said this, there are a few contacts you can poll to get a definitive yes/no answer on the matter.
Here is a pdf document from the Diabetes UK organisation which summarises a lot of interesting information relevant to travelling with diabetes in Colombia. It is in my opinion one of the most complete sources of authoritative information available on the web.
how to obtain insulin section of this leaflet does indeed suggest to inquire for medicine availability in your destination country, by contacting the manufacturers of your brand of insulin. Moreover it mentions the contacts of two Colombian associations you might want to contact either before or whilst travelling:
Asociación Colombiana de Diabetes, Calle 39A, No. 14-78, Santa Fe de Bogotá, Colombia
Tel +57-1-2880777 or 2883420 -- Fax +57-1-3231582
E-mail email@example.com -- Website www.asodiabetes.org
Federación Diabetológica Colombiana (FDC), Calle 94 no. 15-32, Oficina 101, Bogotá D.C., Colombia
Tel +57-1-6359710/11/12 -- Fax +57-1-6359711
E-mail firstname.lastname@example.org -- Website www.fdc.org.co
Your Local Colombian Embassy/Consulate
In addition, the Travelling page of the Diabetes UK website advises you to ask your local Colombian authorities - embassies, consulates, etc. - for information on the topic.
First Hand Experiences on the Web
Prescription and Explanation Letter
There are however a number of first-hand experiences from diabetic worldwide travellers recorded on the web (see here for example). Most of which seem to suggest that with a prescription, and an explanation letter, from your "home" doctor, and a visit to a local doctor, you should be able to get a prescription in any country in which insulin is readily available. If you want to go the extra mile, think about translating the letter and prescription in Spanish.
Insulin Over the Counter in Central America
On the other hand, this post from a Lonely Planet thread on insulin and Central America states that it is possible to buy insulin over the counter in Costa Rica, without the need for a prescription:
I live in Costa Rica and there is no need for a prescription to buy insulin and needles.
This is about the only direct source of information I could find on the web. Of course you should not take this for an absolute truth.
I personally would not trust internet opinions alone, and would not gamble on the availability, and ease-of-purchase, of insulin in Colombia. Hence, be prepared and bring some supplies from home along with the necessary documentation from your doctor back home.