You will find a number of useful resources in the IACCM Resource Library. Without knowing what sort of agile development you are envisaging, it is hard to suggest a specific template, but I suggest you go to this recent guide www.iaccm.com/resources/;
In general, the key point about an agile agreement is that it depends on collaborative working and therefore traditional risk transfer and 'time is of the essence' principles do not apply. The agreement needs to recognise that there will be specified milestones and on achieving a milestone, there will be review and potential renegotiation to support the next phase of work.
You should also consider how or when you might convert from the agile agreement to another form of contract. We use agile because of high levels of uncertainty. Once the uncertainty has been resolved, it may be far more appropriate to convert to a more traditional time and materials or fixed price agreement.