A few forestry associations are still selling Christmas trees:
Árnesingar Forestry Association, at Snæfoksstaðir, until December 23rd, hours: 11-16. See: https://www.facebook.com/snaefokstadir
Eyfirðingar Forestry Association, at Kjarnaskógur, until December 23rd, hours: 10-18. See: https://www.facebook.com/SkograektarfelagEyfirdinga
Mosfellsbær Forestry Association, at Hamrahlíð, until December 23rd, hours 12-18. See: https://www.facebook.com/SkogMos/
Reykjavík Forestry Association, at Lækjartorg, until December 23rd, hours: 16-20. See: http://heidmork.is/