Greenmarkets where you can stock up on holiday greens:

A'moon Farm: Wreaths, Bouquets, Garlands, Princess Pines, White Pines, Cedar Pines, Mountain Laurels, Boxwood, Juniper, and Rosehips (Union Square Greenmarket Wednesdays and Fridays)
Downhome Acres: Wreaths (77th Street Greenmarket Sundays, Columbia University Greenmarket Sundays)
Durr Wholesale: Wreaths (Union Square Greenmarket Saturdays) 
Fantastic Gardens: Potted evergreens, poinsettias, amaryllis (Union Square Greenmarket Wednesdays, Fridays & Saturdays)
Green Life Farm: Wreaths (77th Street Greenmarket Sundays)
Lebak Farms: Wreaths and boughs (Grand Army Plaza Greenmarket Saturdays)
Luna Family Farm: Wreaths and bouquets (Columbia University Greenmarket Thursdays; Stuyvesant Town Greenmarket Sundays; Union Square Greenmarket Wednesdays & Saturdays)
Mountain Sweet Berry Farm: Wreaths, garland, and princess pines (77th Street Greenmarket Sundays; Union Square Greenmarket Wednesdays, Fridays, and Saturdays)
Newgate Farms: Wreaths (Cortelyou Greenmarket Sundays, Columiba Greenmarket Thursdays)
Pe-tal Plants: Potted evergreens, poinsettias, and amaryllis (Union Square Greenmarket Mondays, Wednesdays, Fridays & Saturdays)
Phillips Farm: Eucalyptus Wreaths (Union Square Saturdays, Grand Army Plaza Saturdays)
Rexcroft Farm: Wreaths (Fort Greene Greenmarket Saturdays) 
River Garden: Dried flower wreaths (Union Square Greenmarket Mondays, Wednesdays, Fridays, and Saturdays)
Stokes Farm: Herb wreaths (Tucker Square Greenmarket Thursdays and Saturdays; Union Square Greenmarket Saturdays)
Van Houten Farms: Trees and wreaths (Union Square Greenmarket Mondays, Wednesdays, Fridays, and Saturdays)
Walnut Hill Farm: Wreaths and dried flowers (77th Street Greenmarket Sundays, Carroll Gardens Greenmarket Sundays, Fort Greene Greenmarket Saturdays, Grand Army Plaza Greenmarkets Saturdays, Inwood Greenmarket Saturdays, McCarren Park Greenmarket Saturdays, Union Square Greenmarket Wednesdays and Saturdays)