We had a Van in South Africa for these 5 or 6 weeks our family (we flew forth and back depending on who gotten when vacation, just spent xmas together) was there. Just called some agencys and looked for the best rate and then took that one.
Usually rental agencys are pretty good if they know you will have the car a few weeks, and there are agencys who are specialized in dealing with long term rentals. What is also possible is that you ask a few car dealerships. They sometimes have cars standing around where a few more thousend miles dont matter and that they rent out...longterm might also get you a good rate.