I totally get the worry about shipping, especially with something so fragile. Besides what they mention about San Alejo and Carrera 13, I'd say you should look in smaller home stores in your neighborhood or in areas like Chapinero or Usaquén, where there are tons of decoration and furniture shops that don't have the inflated prices of the big chains. It's also a good option to check out discount stores or second-hand furniture in good condition, because sometimes you find really nice mirrors for less money.
What worked for me recently was just walking around local commercial areas and comparing prices without rushing. It's tedious, I know, but that way you see exactly how the frame and mirror quality are before you buy, and a lot of times the sellers will negotiate if you buy right there. If you end up deciding to go with marketplace anyway, at least look for sellers that have lots of positive ratings and offer local delivery without shipping it from the other side of the country, that way you reduce shipping risks.