Most vets have canine blood donors. When i did my work experience 5 years ago at the vets, there was a grey hound that gave blood, it didnt take very long and as payment the dog was given a few cans of dog food as a thankyou.
if you decide that you want your dog to give blood, remember you wont be paid money but your dog will be rewarded with food!

A standard blood donation in the dog is 450ml (‘one canine unit’) and this can safely be obtained from a 25kg dog; smaller amounts may be obtained from smaller dogs.

Donor animals should be checked by a veterinary surgeon that they are fit and healthy enough before making a blood donation, in which case the risks of donation are very small.

Repeated blood donations over a relatively short period of time can lead to anaemia, and should be avoided unless absolutely necessary. For this reason, after a donation is made and recorded on your vets database, the donor will be blocked from being called via the registry for three months.

An ideal blood donor is a friendly, healthy, clinically normal animal that is not pregnant or has not produced a litter if an unspeyed bitch. Donors should be vaccinated (although not within 10-14 days before donation) and free of infections and parasites, especially blood borne disease.

Dogs like humans, have blood groups and can be blood typed. Ideally, donor and recipient should be type matched. As well as typing donor and recipient, cross-matches can be performed to confirm compatibility, and are recommended where the recipient has had a previous transfusion.

This test involves incubating donor and recipient serum and red blood cells and looking for a reaction outside of the body that indicates an increased risk of a reaction inside the body if the transfusion is given.

to find out about canine blood donors you can visit: Dog Blood Donors, register your dog to help save a pet's life!

OR visit: www.rvc.ac.uk/.../BloodDonorClinic.cfm

hope this helps