Treats are easy to get, as a rule of thumb, the smellier the better. Make sure you cut them into tiny pieces though so you don't spoil his appetite. I personally use hotdogs, buy the cheap tin ones, they should be fine, one hotdog cut into 30-40 pieces should be enough to keep him going for hours (even days!).

Make sure to also start weaning him off the treats as well, so give him a treat with a 'good dog/boy/girl' praise when he has heel'd. Just start giving him a treat every other time, whilst still verbally praising him, over time, one treat every 3 times and increase. This will be better for his diet and for your bank balance!