Hemlock (Tsuga canadensis)appreciates more acidic soil, higher humidity and wetter winters than Front Range Colorado can provide. To learn about several evergreen alternatives, go to: Evergreen Trees.
Note that "big box" stores ltend to order stock from a central facility rather than through individual, local stores. Some plants that show up in a big box store are not well suited to Colorado. On the other hand, on occasion you can find some unusual items not normally carried by local garden centers that can grow here if given tender loving care.