In the UK, here are bereavement support contacts from Dogs Trust (Irish people could also use these):
For those in North America who have lost a loved pet, there is a free (800 number) Pet Loss Support Hotline that is staffed by trained vet students from UC Davis Vet School in California. They also have an extensive array of links to other pet bereavement support lines and websites.
For more information, see:
Support website, message board, chat:
Another extensive resource site to help with bereavement at the loss of a pet: http://www.argusinstitute.colostate.edu/grief.htm
UC Davis Vet School's information on knowing when it is time, and bereavement.
Here's a very helpful sheet from Ohio State vet school on coping with greif and loss of a pet.