There are a few makes around but these are IMHO very good...............................http://www.ruffwear.com/dog_packs
That Website says of their "Approach Pack" holds a couple of litres and that it generates "instant four-legged motivation".
Don't they do anything bigger? I was thinking 25 or 30 litres. Believe me, motivation is not a problem with my four-legged friends.