hit up home depot or lowes. HD sells 42w 2700k bulbs for 10 bucks.
I don't know how many you have or if you're looking to buy as many as you need at once, but compare it to the prices for an HPS system. You might be able to get an HPS for about the same amount spent in cfls. You can get 250w hps systems for $95 or 400w for $150 from HTGsupply. Just a thought.