Art supplies of decent quality are difficult to purchase here - in general, we advise bringing as much as you think you'll need. There are some shops in Bogotá, but anything of decent quality has been imported, and is thus priced more to cover shipping and taxes. Seriously, bring art supplies.
For the time being, US Amazon has been offering free delivery and without duty on some items. We have an address in Bogotá to which you can safely have things shipped, and then someone can bring it up from the city.
Some of the better shops in Bogotá:
Fabriano - located in the Santa Barbara mall in Usaquen Belle Chantraine Panamericana - multiple locations in Bogotá
For paintings, artists generally work on temporary stretchers, and then roll the paintings. Silicone release paper can be handy for lining the paintings.