there are a number of places on line
CarmenWarehouse.com has the best price but very limited color choices, I usually only get neutral colors beige, tan, light and dark greys, and keep white and black on hand.
Also try ebay,
All That Thread.com should have all the colors and the YLI's silk and cotton.
Keepsake Quilting.com has a combo package that is a fair price. I use the orange spool,50wt., 1422 yards.
I just checked All That Thread, their price has dropped a $1 to $8.50 for 1422yds. This may seem high but one spool seams to last forever.