You have two choices in setting resource availability - one by changing the resource's max units over time through Resource Information under availability. However, changing this on a weekly basis is going to be a lot of work. Also, if you assign a resource to tasks that span more than one week, Project will not automatically adjust the assignment units up or down if the availability changes per week.

You can also change a resource's calendar - but again, this is going to be painful.

Explain a bit more why you cannot get a firm commitment from the sponsor regarding resource availability. It is going to be very difficult to manage a project where the resource's ability to work is going to vary by week. For example - if I have a resource assigned at 20 hours for a week - but then the task shifts into the next week - are you going to be expected to recalculate the task duration based upon the resource's change in availability?