I have been searching for a good deal on these for quite a while and stumbled upon one!! Groupon is offering a 8 pack of replacement heads for Philips Sonicare toothbrushes.

They are compatible with the following models.
Diamond Clean, FlexCare, FlexCare+, Healthy White, and Easy Clean models

If you change your toothebrush head every 3 months this would give you a two year supply.

The fine print – Limit 3 per person, +$2.99 shipping rate

You can grab the deal here – Philips Sonicare Toothbrush Replacement Heads 

This is better than any coupon deal I have stumbled upon!